When a chin feels threatened, whether by a person or another chin, they are able to shoot a stream of urine at the offender. This is called 'spraying'. All chins are capable of doing this, but not all will exercise this ability. A chin will stand up facing the perceived threatening party to accomplish this. It is amazing how accurate they are at hitting their target. It is also amazing how far they can spray. They usually do this very fast, often before the intended victim has time to move. There are times when they will give a warning stance, which is standing up fast and moving their pelvis slightly forward, but not actually following through with the deed. If he does this, heed his warning and leave him alone so he can calm down.
Try to replay in your mind what actions you took to make him feel threatened and obviously try to avoid doing them again. It could be something as simple as moving your hand too fast towards him which he perceived as an attack. Usually a chin that is so quick to spray has been handled roughly by someone in his past. Please show extra patience if your chin should spray you. Yelling at him will only make you look more like a threat to him and never hit your chinchilla under any circumstances. We have had chins come to the rescue that would spray or threaten to spray if we even came near the cage, let alone try to open the door. Over time and with a lot of reassuring, soft spoken words, we did gain their trust and they stopped spraying. We can not emphasize enough how important trust is between you and your chinchilla.