Andy Burns explained :
Rule two. Always check the automatic windscreen wiper setting is off before
turning on the ignition, or the sensor may interpret scraping the windscreen
or pouring tepid water on the windscreen/blades as "rain".

My auto-wipers cancel themselves, even if left on auto, fast or slow,
if the ignition is switched off. The wiper switch has to be moved to
reselect them. A constant annoyance and I've always wondered why, but
frozen wipers seems to explain it doing that, though I suppose the
system could just check the temperature outside before inhibiting them.