There are several posts with problems such as you have noted in the archives, and probably as many causes. Several locations don't do well when wet (so don't use a firehose). The headlight is not watertight and contains many connections, the sidestand safety switch may fault when wet, the connections under the tank may short when wet, the engine cut-off at the handlebar, the electical connections adjacent the key switch are not watertight.
Maybe try hosing the bike down while running to see if it dies when water hits a particular area, then troubleshoot more indepth.
Wash carefully, using a bucket and light spray, and hey, ride more, don't wash so much!
