We had (and unfortunately still have) a nasty troll, that attempts to downvote all the questions and creates sock accounts for that to escape suspensions. To stop them, we had to employ some limitations. Voting limitations will likely stay in general, although we can and will improve the exact mechanism.
The process currently works like this:
You have at minimum N votes (defaults to 5, can be changed per-site). To that we add the number of reputation points you have minus 1 (your initial reputation). This results of the maximum amount of votes you can use in a rolling 24 hour window.
Update: There was a bug with calculating the vote limit, which has been fixed now.