Why is my horse sometimes grumpy?

I loan a 19 year old thoroughbred. he is a lovely behaving horse, 16.2hh in height and not a bad bone in his body. i ride on him sometimes, just trotting and walking and schooling him in the field doing circles and changing reins. i groom him at least five times per week and tie him up on the other side of the fence from his two field mates, both males, one 15hh and one 16.3hh. the three horses have rivalry to see who's boss. my horse usually just amuses himself with crib biting on the fence, but recently he's been putting his ears back and jumping to bite on one of the horses who are tied up opposite to him. he lets me pick his feet out, groom etc, but he keeps turning his head quickly with his ears right back, as if he is not happy with me brushing him. he keeps biting the other horse and stomping his feet. why is this? just to note, we missed out a few days of grooming as we were busy, three days at the most, but this shouldn't have an affect on whether he likes to be groomed. usually he loves having a pamper, and still lets me lead him in and out of the field and is mostly well-behaved, but just occasionally a little grumpy. can you help? thanks.