you obviously just haven't given this question the thought it deserves then have you!!
1.its too hot to wear them
2.He forgot them
3. it will enable him to run faster as he will be more streamlined
4.he got shot on them once and believed they were bad luck
5.he may have nice legs and want to show them off
4.His new sponsor is anne summers and he is sporting one of those sheep pouch thingys
5.he was told to take them off due to excess baginess or too many layers
6.he has become a naturist
7.he doesnt know he's not wearing them
8.he wants to know if anyone will think it will help them play better and then copy him
9.he lost a bet
10.he won a bet
11.it is a bet
12 he sold them to pay off a bet he lost.
13.he is in the middle of a game of simon says and simon said take of your pants.
14.he is wearing pants and you have taken too many drugs
15.he is trying to confuse the other team.
16.he's trying to get a tan
17.Someone sold him some invisible pants
18.some b****rd took too long in the portaloo and he had an accident in them while waiting.