Bunny suddenly starting nipping etc

hello,
I have two indoor bunnies. Had the one (Bracken) last December who has been quite a poorly chap since I got him from the rehoming centre. Acquired the other one (Monty) in March. He had a rough start in an overcrowded hutch and has one dodgy leg. He came from a lovely second home to me. They have both gotten on fine no problems, one day Monty started scrabbling on Brackens side particualarly when he tried to eat, but two days later Bracken got pneumonia, so I thought it was that Monty had been trying to sort of chase him out the warren type thing. Since Bracken got better, no problems at all. Until the last couple of weeks, Monty is going through the most amazing moult (that's the only change in him) but when I've been stroking him as normal he keeps grunting and nipping me, so I am covered in bruises. He has also started humping Bracken and diving at him grunting, again especially when he tries to eat. Bracken is so placid he doesn't do anything or just runs off and sits in the litter tray. It's really upsetting me and I don't really know what to do. Other time Monty will just settle down and snuggle with him as normal. At first I thought Bracken was going to be poorly again, he has just got over an ear infection and constantly has the snuffles anyway, but touch wood he hasn't. So not sure how to react when I see Monty doing it? someone suggested flicking him on the end of the nose, but not sure I like the sound of that.
Any thoughts greatfully received.

has he been done? the aggression can often be associated with sexual maturity, my rabbit was the same and since i had have him done he is tons better...... im not expert but i had a similar situation with Smudge so hope it helps x

Could it be that he is in pain somewhere? Have you had him checked at the vets?

I certainly wouldn't flick him on the end of the nose, that would just make him scared of you and would achieve nothing.