It happens every once in awhile. I don't know why either. It's like the metal of trier expands. I have had to replace this twice because I broke the handle trying to get it out. I am always able to get it out though...once I had to cool the roaster down and hammer the tip of the trier to get it loose. I wish there was a pattern so I could fix it.
Just go get some emery cloth from your local hardware store, around say, 120 grit, then wrap the cloth around the lip of the trier where the two metals fit together, squeeze and rotate for a minute or so, put back in and repeat as necessary. Just don’t go so far as to make it, a sloppy fit. This should fit the problem. Most likely theres a burr that the cloth will remove. It may also be in the trier hole, so just go light working with the hole.