I just noticed that 2 of my reviews from several months back are gone (they were up for a while, though). Does it mean the authors deleted them or they get pulled for some reason? Is it a normal occurrence?

Please don't take this as a complaint- I'm simply curious why it happens.

Thanks!

It is actually difficult to get a review removed, even by the reviewer who wrote it. Once submitted it is TER property, and they don't easily remove them. There has to be a really good reason. You might be able to ask admin. They may or may not tell you the reason. If you remember the handles of the reviewers, one thing you can check is if only your review is gone or if all the reviews he wrote is gone, or if his whole handle is gone. Another thing to check: Is it possible the same reviewer wrote more than one review of you? If so, the new one replaces the old one. There can only be one per reviewer.

Some possibilities on the extreme side:
The hobbyist got outed or arrested, and asked TER to remove all his reviews and posts.
The hobbyist got caught writing fake reviews and then all his reviews got called into question.