This sentence sums it up:
‘MPs, angered by the latest instance of Johnson’s erratic relationship with the truth, have been eager to oust a leader many now regarded as an electoral liability.’
Read the article in The Guardian here.
Johnson had become a liability to his own party.
The Pincher scandal is a minor scandal, given all the scandals that surround us. They were waiting to give Johnson the final Blow, Pincher just helped them, Pincher was a valid excuse.
Shrewd? Maybe not so shrewd. Also in politics, luck does play a role, as every gambler knows.