My horse does this every once in a while. One day he'll take a few steps away, next day it's a few more steps and next thing I know he wont let me catch him.
What I do is spend a few days working with him (generally just takes the weekend). What this entails is putting him in an area small enough he really can't take more then a few steps to get away. I then go out repeatedly to catch him up, give him a few strokes of a brush and walk away again. When he no longer tries to step away I increase his area, rinse and repeat until back to their normal self in the full pasture. Most horses that have already been taught something (not running, standing still, etc) don't take long to whip back into shape again. It's more of reminding them how to behave instead of teaching them to behave.
Generally just use a temporary electric fence as they are easy to increase in size with a few minutes worth of work.