I reckon he had the ladder at a ridiculous angle in order to get the top to rest on the sill. Went up it and it buckled and gave way.
If you're going to window clean off a ladder, why o why do people not use a double a frame ladder? They are designed and built for the job and are much safer.
For the record, I'm almost 100% wfp but still use a ladder occasionally - I would never window clean with an open ended ladder, even if its a trade rated ladder.