I concur with Mr G's diagnosis of cause and effect.
I use a different pole hose (Sureclean Systems) and also have a tap at the brush head.
I have noticed the pole hose swelling recently in this warm weather.
And I adjusted the pump pressure screw because I was having problems with water spurting from push-fit connections due to being over pressurised.
Yesterday half way through a window clean I must of spent 20 minutes having a conversation with a customer.
During that time I noticed the pole hose had swelled quite considerably probably to twice it's normal outside diameter.
So I believe it's a combination of heat softening the hose, possibly too much pump pressure and the tap at the brush head.
The majority of the time I never see this problem but with this hot weather it does happen.