
Originally Posted by
vBET
Mostly common is that people forgot that they made changes in robots.txt 2-3 months earlier (yes Google reacts in such time - it can be faster if you have super high Page Rank). Also sometimes people look on results in Google search - where is shown only approximated number of indexed pages, not the real one. So approximation could change to better one and it doesn't mean that any of your pages was removed from index. Also - Google is huge and have lot of databases which from time to time are updated, replicated and so on - this can be only effect of working Google database algorithms. Also Google is changing it's indexing algorithms and i.e. it is possible that some of your already indexed pages are now considered as duplication of content (i.e. most member pages are in 99% same) and removed from indexes.
As long as you do not see large number of errors in your webmaster tools you can assume that it is just how Google works. But if you see errors - then it is on your side and then you should react.