Trotting

There was a horse gathering down in County Durham today and we got caught up in it on our way home.  Fortunately we missed the main trials and generally speaking every outfit we came upon was well behaved and drove sensibly. 

Sadly it only takes one idiot...  As we approached a roundabout he decided to briskly weave from lane to lane so that none of the following long line of cars had a clue where he was going.  No hand signals, no acknowledgement that he knew anyone was behind and every sign that he didn't care.  He zig-zagged between both lanes right round the roundabout and then, looking round he grinned and went back from whence he came.  Why do they do it?