While no one is publicly commenting on what finally happened to tip the scale and trigger the dismissal of Jeff Cassar, I believe it can be summed up rather succinctly by the following series of posts on twitter:
Or, he took it in December 2013. That's a deficit of about 0.3 xG per game, and 1 shot per game
— Kevin Minkus (@kevinminkus) March 20, 2017
To put it bluntly: If you do not put the ball in the back of the net more than your opponent you will not win games.