Sometimes when a person is learning a new language they get stuck in the wrong habits. Their whole life, they end up speaking with a nearly unintelligible accent, or they use grammar that is not correct or hard to understand. I see the same thing happening with improvisers.
Sometimes an improviser will learn that blocking their partner get a laugh, so they will be reinforced and continue doing it until it becomes second nature. It becomes hard to self-correct the behavior because so many good emotions have accompanied it even though the effect on the scene (and their scene partner) is not desirable.
Even the smallest of errors can accumulate. Even behaviors which are not really errors can accumulate into a rigid, bony structure that we get stuck in. It requires constant vigilance to protect ourselves against this kind of fossilization.
One thing that helps is identifying what kind of scene we're in and what kind of character we're playing. I have a tendency to err toward high-status characters, often those who are jerks (perhaps because I never really get to be this way in life... or perhaps quite the contrary?). Once I realized this, I made a conscious effort to chose to play more low-status, vulnerable characters.
Improv is an act of spontaneity, but as with all acts that are repeated, we risk making them dull if we don't seek to make them new. Then again, we risk missing our beats and losing our character to our everyday habits if we do not practice repeatedly. We want to find a healthy balance of repetition and variation, both within the scene itself and for our overall practice of the art.