I found a bulletin, PI0636, that relates to this. It's only listed for 2010-2011s, but it appears that later models may have the same issue. It instructs the dealer, before replacing the liftgate switch, to first check with the customer to see if it has happened without the doors locked. Otherwise, it could be just an inadvertent button press on the fob.

When this happened to ours, this factor was irrelevant because the exterior switch was completely dead. While waiting for the part, however, it did decide to open while we were moving. They therefore pulled the fuse until the part came in, as I described above.