Here is the original post, where she does in fact state her horse did not have issues before.
"I have to use treats to get him to open his mouth" - have, not had, meaning she has to do so now, not before.
This is why I believe it's a dental issue, not training. Even biting while she's tacking up - could be back pain, muscle soreness, dental related, stomach ulcers. A vet or chiropractor will have to come out to be sure. If all health related issues are ruled out, then I would begin different training approaches.