Horse that has started spinning/bolting?

I have a 18 year old gelding who has suddenly started spinning/spooking when out on the trail. He always spins to the left. He eyes are clear, doesn't appear to be sore on his back, same saddle, bit, collar, same pad (expensive pad). The only change is that he was started on senior food and being fed about 7 lbs. twice a day. He also gets a rice bran mix. He doesn't like leaving his stable mate, but two of the spinning events was when she was with him; so that's not the problem. He has always been a little tense, but hasn't bolted around like that since he was 3. Any advice would be helpful as I a beginning to question my "seat or balance" on the horse, which until the last few weeks has always been good. I also remount him when I've gone off. No serious injuries yet, but unless this can be corrected, I can see them coming!