Maintenance for the week of April 6:
• [COMPLETE] ESO Store and Account System for maintenance – April 9, 9:00AM EDT (13:00 UTC) - 6:00PM EDT (22:00 UTC)
Seems to happen when a PvP zone starts to get near full, it will occasionally just lock you out mid-load. Best explanation I have for it, just keep relogging until you can get in, that seems to be the best method.