I don't know if anyone has had this happen before, but my old faithful who is 15, and the sweatest gelding ever has seem to become a little aggressive these days.
He has never in his life for as long as I have owned him offered to kick or bite, and has never pulled back.
On Saturday, I could not catch him (which is weird), I cornered him finally, and he acted like he was terrified of me and then actually fired at me with both feet missing me by a very small margin (this is a first). I finally caught him and took him to the jackpot. First thing when I tied him up at my trailer he pulled back (another first). Later I walked away from him, and he actually pulled back and then came forward striking at me with his front feet and squeeling (another first). I was so shocked. He was good when I rode him. I moved him to my sisters Saturday night because the farrier is coming there, and when I went there last night he ran up to me and was himself looking for crunchies. He is not spoiled, and has always been a gentleman.
Someone told me if I have alsike clover in my field they have heard of this causing nervouse, aggressive behaviour. I did just move him into that field, but he has lived there ever fall and winter for the past 7 years.