Check body, podiatry, saddle, teeth then yourself-has he been over phased, frightened, lost confidence?. If something out of the ordinary happens rarely indicates a horse rationally decides that THAT is how he is going to evade.
Some show jumping coaches use to tell their students to canter on a voltage/circle in front of the obstacle. It's easier for horse to just jump over than work on a circle when they have to collect themselves. Ofcourse this doesn't work on every horse certainly.