One minute they were 10 points clear and promising to show the Warriors a clean pair of heels on their new synthetic pitch. The next they were counting the cost of the Warriors clicking into overdrive, Seymour's second-half hat-trick in 20 minutes causing enough wreckage to strip the Leinster men bare without as much as a losing bonus point to their name.
Seymour's shake, rattle and roll act made him the first Scotland wing to score that many in the Guinness PRO12 and only the fourth of all-time following Mike Mullins for Munster at Caerphilly in 2001, Lee Abdul for the Blues against Ospreys in 2003, Jamie Robinson for the Blues against Ospreys the following year.
Leinster are the first Irish team to be on the receiving end of such a quartet. The irony will not be lost on their executioner-in-chief, a player who learnt the game in Ireland.
Seymour's Anglo-Scottish parents made the long haul from Tennessee to Belfast via Dubai for work reasons when the young Tommy was nine years old. He began playing rugby at school and by the time he left had joined Ulster as a young professional.
It didn't quite work out. The sum total of his two seasons at Ravenhill amounted to small numbers - seven appearances in the Guinness PRO12, one try. It took a move to get Seymour going, from Northern Ireland to the west of Scotland, from one famous shipbuilding city to another.
In Glasgow, the Warriors transformed him from the fringes of the supporting cast five years ago into a headline
act for club and country by virtue of his mother, Sue, being a native Scot.
Seymour, perhaps more than anyone, gave the perennial contenders the cutting edge behind their majestic title triumph rounded off at Munster's expense in the final, staged, fittingly enough, in Belfast.
The new season may be barely a fortnight old but the Warriors have already given a clear indication that they intend to regain their status as the Guinness PRO12's No. 1 team. In successive weeks, they have hit the champions for six tries in Galway and followed with five more against last season's beaten finalists.
It sends them to Cardiff on Friday night with a perfect 10-out-of-10, a feat matched by the last Welsh winners of the tournament, Ospreys. They followed their thumping home win over Zebre by showing that anything the Warriors could do in Galway, they could do too.
Connacht could have done without another painful reminder that while getting to the top is one thing, staying there is a very different matter. In not much more than the blink of an eye they find themselves 10 points adrift, a startling fact that underlines the ferocity of the competition.
In bolting Usain-like out of the blocks, Warriors and Ospreys are the first teams to take maximum points from the two opening rounds of the Guinness PRO12 for three seasons. Munster did so in September 2013, beating Edinburgh at home 34-23 and Zebre away, 43-21. Of that Munster team only six remain - Andrew Conway, Dave Kilcoyne, John Ryan, Donnacha Ryan, Tommy O'Donnell and CJ Stander.
Cardiff Blues, off to a similarly spectacular start in 2011 before finishing amongst the also-rans, have dropped just the one point in beating Edinburgh and, more significantly, Munster in Cork last weekend. That they came from behind to do so made it all the more satisfying, Steve Shingler's conversion of Dan Fish's try nosing the Blues home by a point.
Their improvement under head coach Danny Wilson puts them in a position to go clear at the top, if only for 24 hours. They open Round 3 on Friday night at home against the Warriors, a match of such consequence that it ought to pack Arms Park to the rafters.
For the Warriors, it means switching from one artificial pitch to another and the prospect of more fireworks from Seymour. For the Blues, it brings a golden opportunity to show their long-suffering fans that they have a squad with the stamina to go all the way over the most hazardous of courses.
Wilson will have made them aware of the dangers, not least that posed by Seymour's mastery of the intercept. He demonstrated his predatory knack at Murrayfield two years ago in rapid succession against the All Blacks and Argentina, and again last week to equally devastating effect.
Ulster, five-try winners over the Dragons in their opening home match, fell one short of a second straight bonus-point win against Zebre in Italy. Director of rugby Les Kiss, no doubt aware that ten years have elapsed since Ulster won the PRO12 for the only time, will aim to make it three-from-three against another Welsh opponent in Belfast on Friday evening.
Scarlets, leaders for a long stretch of last season before falling at the final few hurdles, make the trip in search of their first point, never mind their first win. Their failure to score more than one try hitherto is all the more surprising given that their backs at Murrayfield last week featured an array of Welsh internationals - Liam Williams, Jonathan Davies, Scott Williams, Rhys Patchell and Gareth Davies.
The challenge awaiting them in Belfast will be no less daunting than that facing Benetton Treviso against Ospreys in Swansea on Saturday night. The two other teams beaten in both opening rounds - Zebre and Connacht - meet at Stadio Sergio Lanfranchi in Parma.
Munster, licking a few wounds inflicted by one Welsh team, face another in Newport Gwent Dragons at Rodney Parade on Saturday.
A fortnight into the season and the PRO12 is rocking, thanks in no small way to Tommy Seymour. It can be but a matter of time before Nashville invites their hometown boy back for a gig at the Grand Ole Opry…
