Lucy Balmer: he pinned his ears, switches his tail but there is no flies. Sometimes he even tries to nip me too.
It may be possible that your horse has Lyme disease. My horse was very naughty and picky when she had it so get him tested.
"Naughty" horses, or rather horses that are unhappy about something and are trying to get this message across to you through their only way of communication - body language, will usually have an underlying problem of this unwanted behaviour. It's likely that he is sore, in pain or is beginning to have a lack of respect for you, but since you've said this unusual behaviour is not normal it's likely to be down to some physical discomfort.
When you say he's acting "naughty", what does he do exactly? Does he try to nip? Swish his tail? Pin his ears?
I agree maybe flies, whats he like being ridden?
Check to see if he has any cuts or is being irritated. If not "massage" him and see if he is sore, that could be another reason. If that's not it he may just be acting naughty and will need discipline. One of the school horses at my barn is like that and the little kids just hold a crop in their hands, they don't hit him with it but just very lightly tap his neck if he starts to act up
maybe his skin is being irritated by the flies??
I would look to see if he has a cut or scrape or even a scratch just in case if your going over it and that's why he is being naughty.