I don't know if it was ever completely articulated what was really going on there. At least not the sudden hideous mutation he had at the end of the Thing series.
Ben went to Battleworld during Secret Wars 1 and decided to stay there after the war was over. Beyonder's world was built to fulfill human desires I think. While Ben was there I believe he realized he could control his transformations and go back and forth between Ben Grimm and the Thing. During the Thing series it turns out the magic of Battleworld had created a second Ben Grimm and a love interest for Ben named Tarianna and pretty much everything else left there that wasn't there from Secret Wars. Duplicate Ben was evil and during a battle evil Ben was killed by Tarianna and vice versa. While Tarianna (just a figment of his mind + Battleworld juju) was dying Ben explained that for years he had a mental block that prevented him from being human (and that always caused him to return to being human despite all of Reed's solutions). The mental block was because in Ben's mind Alicia Masters loved him as the Thing and didn't really love him as Ben. Between their relationship problems (which he alludes to even before catching her with Johnny) and Battleworld the mental block seemed to be gone. But Tarianna killing the evil Ben and Tarianna being a part of Ben made Ben come to the conclusion that his subconscious was going to keep him as a Thing forever.
After Ben finds out about Johnny and Alicia he has a conversation with Reed. Reed tells him that ever since FF #245 when Franklin considered turning him back to Ben Grimm but chose not to he's known that the reason Ben stays the thing is because of that mental block where he thinks Alicia only loves the Thing. Franklin had been worried that curing Ben would have ruined his relationship with Alicia and Ben couldn't have handled that. Reed reasons that the relationship breaking down and Battleworld was why Ben was able to control the change and reasons that now that Alicia is out of the picture Ben should be able to control the change at will. Ben is upset because he killed the human side of himself on Battleworld so he's back to being unable to control the transformation again.
To me there was no real explanation as to why his mutations were getting worse when they did...it was some time after killing evil Ben.
1. Ben later discovers Sharon Venture looks just like Tarianna the woman his subconscious supposedly conjured up.
2. Sharon gets turned into a Thing and Ben into the pineapple version in Englehart's run.
3. When Sharon first meets Reed she takes Reed to task for telling Ben he could control his transformation at will. Saying that giving Ben that false hope made him more miserable.
4. The Thing lost his powers later that same issue. My belief has always been that it was easier to depower the more powerful pinecone version that it ever was the standard Thing form.
5. Later Sharon betrays the team to Doom so he can depower her as the She-Thing while in a gift of the magi type twist Ben turns himself back into a Thing so she won't be lonely as She-Thing.
And of course the thing I've always wondered...does Sharon have a mental block? Is the reason she looks like her ex-boyfriend because she can't get over her ex-boyfriend?
I've always believed that Sharon has a great many psychological issues that even predate Ben.
But I think Ben's problems are psychological, too.