I can’t wrap my head around how bad Cory became over the course of a year or two.
Likely injuries. There are many players who start to perform poorly at his age, which isn't old, but it's older than league by 5 years. It's just that the players who perform this poorly at his age, were never as good as him before they started to perform badly.
Jimmy Howard and Karl Lehtonen fell off at similar ages. Howard is actually having a good season, but between 2013-2014 and 2017-2018, he had one isolated and random good season and I think he only played like 30 games or something that year. The rest were not even average. Lehtonen was a very high goalie pick and he was very solid, but was he ever as good as Cory? In his pre-NHL career he was, which caused him to be drafted so high. As far as the NHL goes? Definitely not.
And I'm not gonna count Cam Ward, as Ward has been a bad goalie for more of his career than not. He just had a few good seasons between 2008 and 2012. His pre 2007-2008 seasons were as bad as his post 2012 seasons have been. Even that year he won the Conn Smythe, he was brutal in the regular season and he still sucked the next two years. That was a random 20 game hot streak from a young but not very good goaltender.
Carey Price certainly seems like he's also going through something similar right now and is a year younger than Cory. And it looks like it started a year later, which would be 30 years old, just like when it happened to Cory. Holtby looks like it might be happening to him and he just turned 29.