PCV hose blew from engine

Hi.
I recently purchased the vehicle listed above second hand.
It was shuddering on cold start when taking off and also around the 80kmph mark if I let off acceleration and then tried to accelerate again.
I changed transmission fluid (was brownish but no metal flake etc ) and
Added an anti shudder.
I was changing engine oil and filter today which seemed normal procedure.
After I had refilled oil I then wanted to change fuel filter.
To do this I was advised to remove fuel fuse, start car to relieve pressure in filter for easier removal.
I took the fuse out, started the car and immediately heard a very loud bang and white smoke. Turned off ignition immediately and inspected motor and found the PCV hose at the top back of the engine had blown off completely.
What has caused this to happen?
It started and ran okay after transmission fluid change.