I'd like to know too. A wild speculation on my part, but it may have to due with Caesar's relationship with Brutus' mother Servilia Caepionis. She and Caesar are well known to have had a decades long romantic relationship and perhaps Caesar either saw himself as a father figure or was the actual father to him. Maybe Brutus had a personal grudge against Caesar like dishonoring his mother some way or may be he stabbed him in the groin as a symbolic gesture to renounce Caesar's parental relationship with him.