It normally stems from some confusion and jumping to conclusions.
Maybe a local window cleaner has dropped an area or a street, or a clutch of houses; another window cleaner hears about it and somehow thinks that the window cleaner who used to clean these has stopped window cleaning altogether - and without checking it out properly - goes canvassing.
Happened to a local window cleaner around here about 10 years ago. It's an easy mistake to make if the canvassing window cleaner is stupid.