The issue is arena caps, and it has been discussed extensively. Do a search for arena caps, just in the last few months, and you'll find lots of information including stuff like:
-Caps are fluid, meaning they are constantly changing
-Caps are linked to each other, if one arena's numbers reaches a certain percent (70%)? Then the other cap will increase.
-HTC does this for game play reasons, though you will find players on both sides of the issue on this one.
-They work most effectively at high volume times, so if you play at a time with less people or at a time where people are generally leaving the arena, you might find yourself in an unfavorable position if the arena you want in is capped. I believe HTC has tried to help that problem by removing caps after 12:00AM CDT.