Same problem I had, the rear wiper was leaking water onto the multi plug causing a short in the circuit which then caused the front wipers to come on.
check for water around your rear wiper plug. I bought a new wiper (thanks Dave

) and had no problems since.
Your lucky theres no grit on the roads mine happened in winter and I pulled the fuse so no wipers would work, by the time I got home from work I could barely see out the windscreen because of all the crap
