The tile in my entrance way buckled. A contractor came and found no signs of structural issues or water. There was no dampness to the backerboard and no cracks in nearby walls or joists. He couldn't figure out a reason. Any thoughts?

Sounds like tenting. Look at the edge of the tile field. There's supposed to be an open space or a soft joint of caulk. This prevents the lateral pressure of your walls expanding and breaking loose your tile.
I'll guess that they grouted between the tile and wall. Is the thinset under the tile completely sheared off the floor?

There is no thinset at all left on the backerboard. Will this recur?

The lack of thinset on the backerboard says even more so towards tenting. It gets sheared away by the lateral pressure.
When you reinstall; leave a 1/4" gap at the perimeter. Leave this gap open, or caulk it. It's preferable to leave it open and cover with 1/4 round shoe molding if possible. If you need any help doing anything with tile; go to:
http://www.johnbridge.com/vbulletin/index.php?referrerid=27552
You can get help with any problems here. They also have a reference "Liberry" to help as well.