Bucking and Using Marks

Mr. Boatwright,

Hope you're happily out of the Thanksgiving food coma. I had a couple questions:

1) My gray horse has started into some bucking recently. Thus far it has been when something scares him (specifically behind him) when we're moving fast. He's always been a little watchy about things, but these are the first times he's decided to really buck about it. I've used circles to correct him away from whatever object started things and then let him rest near the object but I'm not sure how clear that is to him. What are some ways you might address this? Should I try to have someone introduce things behind him and make him face up as a way to get used to things in that spot?

2) Using marks. I took the same grey horse out west last year and rode a bunch of pasture (at least a bunch to me) and came out of the deal with some using marks. I attributed them to poor saddle fit and have since switched saddles. After reviewing the marks I noticed that one side is significantly larger than the other. I'm thinking that may be due to me leaning and putting more weight on one side than the other.  Do you think that's a likely explanation?

Richard Boatwright