Horse bucks on trail rides?

I have an 18 year old QH who has been ridden many times on trails by previous owners. Today was my first day out on a trail with him and it was suprising! I have owned him 7 months. We went out with 2 mares and another gelding and my QH decided he HAD to be first and was chomping at the bit like crazy. He is never spooky in the ring or around the yard but today when another horse started to get close he started to buck and when the gelding gat near he actually reared. He has never bucked with me before. I thought my lazy old QH was going to take off running with me and throw me off. Any ideas why he may be acting this way. I was riding english with a lifter bit that I normally use. He started english 3 years ago before that he was apparently western.