User:XLinkBot reverts based on rules, which are stored as regular expressions in its database. These rules give a more precise reversion than a general rule (as sometimes not the whole domain is to be reverted, but only certain parts of it, e.g. images on photobucket and imageshack are reverted, but not additions of the domain itself).

When reverting, XLinkBot tries to extract the link that matches the rule from the last revision of the page. This list should include the link that you added or changed. This should give more precise information on why it was reverted.

Some commonly reverted sites are:

rule (regex) domain why is this reverted?
[A-Za-z][0-9]{2}\.photobucket\.com/albums/.*\.jpg) photobucket.com image share site, see also the copyright policy
img[0-9]+\.imageshack\.us/img[0-9]+/.*\.(jpg|jpeg|png|gif|bmp|tif|tiff|swf) imageshack.us image share site, see also the copyright policy
img[0-9]+\\.exs\\.cx/img[0-9]+/.*\\.(jpg|jpeg|gif|png|svg exs.cx image share site (actually this is imageshack), see also the copyright policy
\bblogspot\.com blogspot.com blog site (See links normally to be avoided, point 11, on our external links policy, often our conflict of interest guideline is also of interest here.
\bpetitiononline\.com petitiononline.com petition site, wikipedia is not a soapbox.

Note: this is not a complete list, but it should give a feel for which sites are on XLinkBot's blacklist, and why sites tend to be on that blacklist!