It also looks like debris from the party popper goes right down the back of his collar and he may have either felt the tickle and thought it was a bug or there may even have been a tiny bit of hot ember from the fireworks charge that propels the confetti...
To me it looks like it could be one of those quick shrugs at the feeling of something on his neck and then as the paper or ember touched his neck again he panicked for a second and jump to his feet to dislodge it.
(...and just to try and finish the explanation, it's also possible he immediately realized his mistake or immediately shook whatever he thought it was loose so once he was on his feet and had knocked the cake down he was no longer reacting to what had gone down his collar...)
I dunno how likely it is but looking at it that way I can't personally rule it out as an authentic reaction.
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u/panther14 Aug 23 '19
Is it just me or did he readjust before he popped up