There is no automatic disabling of tags (yet). I can only see two reasons (and maybe what Target mentions, which I wasn't aware of).
1) When implementing auto-challenge-checkers we found that some scripts took several gigabytes of memory for some users. We then disabled the tags which had issues. I think there has been 2 unique tags disabled in that phase, one of them disabled once and one of them disabled twice. This is 1-2 months ago. So I doubt this is the reason.
2) If someone else owns the script, he/she can edit your tags as well, including disabling them. If that is the case, I would suggest that the script owner leaves a comment in the tag comment field when doing so. That he hsa done it could be because it isn't correct, or that it fails to run on some users and turns up on his "My issues"-list.
Adding the relevant tag id to your post could maybe help us to figure it out.