Even if it was in the middle of summer and the sun was high above, Shikamaru chooses to wear jogging pants today. He can't quit on the plans he had with his friends. They'd just end up crowding the den and his mom would berate him for not cleaning up when they had visitors. So basketball it is.
As to why he chose to wear pants, knowing how hot he will be and how unusual it was for him to make things harder for himself... Well, he has Ino to blame for the hickeys she left all over his thighs and legs last night.