Big problem or not, a bugs a bug. Probably not a priority though, which is fine, I doubt a single game will ever be altered by this if it's never fixed.
If I had to take a guess as to why, looks like Trundle started his chomp on a minion and the minion died before the spell finished, for some reason that put his next attack in a weird state and bypassed whatever they did to prevent him from biting turrets. Obviously they just need to add a few lines of code, right? If that's correct, again, I can't imagine this being used to affect the game in any feasible fashion. The amount of setup required to do this reliably (even assuming it is reliable to replicate) wouldn't result in higher tower DPS as opposed to just smacking it.