Probably because your ignition electrics are shorting to earth. Try recreating the conditions on a dark night and get someone to watch where the sparks are tracking from.
You may find a cracked, perished or worn cable. If you cant find anything, try an aerosol of ignition sealer spray on all the ignition circuits
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix motorcycles.