Maintenance for the week of January 5:
• PC/Mac: No maintenance – January 5
• NA megaservers for maintenance – January 7, 4:00AM EST (9:00 UTC) - 10:00AM EST (15:00 UTC)
• EU megaservers for maintenance – January 7, 4:00AM EST (9:00 UTC) - 10:00AM EST (15:00 UTC)
It think it happens when there is some type of crash on the "shard" you were on. When you login again your characters likely "moved" to a working shard but a few minutes of loss in progress seems to be the result.
This has happened a few times for me as well, but compared to the loss others have experienced, I've been grateful it is only about 5 minutes of loss.