Two ideas involving bullying and tickling that live rent-free in my head:
1. Being the casual victim of a typical bully - shove you into lockers, call you nerd, tease you for your interests or intelligence - and one day, they find out you're ticklish. Naturally they're not gonna let this go, and - being physically stronger than you - it doesn't take much for them to pin you to the floor and absolutely wreck your shit with tickles. By the time they're done - and you're an exhausted ball of giggles - they declare that this was fun, and you can expect the same to be occurring much more often.
2. The reversal of the above. You've lived under the thumb of your bully for long enough, that when you get a peek into a potential weakness of theirs - and it's being ticklish of all things? - there is no way you're turning a blind eye to things. Time for some well-deserved payback.