Sores in rabbit ears

I have a 13-week-old rabbit who, a day or two ago, developed these sores in the same locations in both ears -- 3 in one ear and 2 in the other. This morning, I noticed that his cage mate has started to develop them. The first guy went to the vet this morning and tested negative for mites. Anyone have a guess as to what else these sores could be? No signs of aggression, itching, shaking of head, etc.

Looks like stress to me.

What did the vet say?

Having two males in the same cage can be problematic as they become adolescence.

doesn't look like the usual mites/infection we get here (which starts as crusty before it gets red, and starts way down low and only becomes visible in the outer ear like that when things are advanced).
It looks like the kind of place where scratching might cause a wound, but i can't tell from the picture if there is liquid in the sores or if they're open. But if there was itching it sounds like you'd see it or the related signs.

13 weeks is adolescent, as Anne mentions, IIRC that was right around when I had to separate my siblings because the head-humping business got to be too much.
What did the vet think?