It could be any of those you mentioned, but most probably, it is them fighting ... especially, if there is a female in the same room with them. If this is true, I would separate them a.s.a.p., before it gets fatal for one of them. I had a father and son that lived happily in the same cage for 9 months, then a female in another cage went into season and they fought for the right to breed (survival instinct) even though neither had a chance to get to her because she was in another cage ... in season. The father almost killed the son. I do not like housing males together where there will be a female within smelling distance ... it is dangerous for the males.
On the other hand ... if there is no female present and you have never seen them fight ... If a chinchilla has a sore or nick on the skin, the other chinchilla will usually help heal a sore by removing the scab and cleaning it ... sometimes several times in the same day, but usually only once or twice a day. Sometimes they can get a little to zealous and make it worse.
I would keep a close eye on them or maybe even separate them until the sore is healed. Neosporin is great to help heal chinchillas.
Jo Ann