A few weeks ago while moving items between characters, I noticed something odd. One of my medium collectible bags was listed as "Bound to Character." Now before anyone asks, I bought it from the Erstwhile Emporium in House P, and had moved it from another character a few days before. I figured it was some sort of shared bank status bug, and since I could easily replace the bag, thought nothing of it.
Today, however, I found the apparent culprit which caused the status change: a set of 4 Ethereal Keys. When removed from my bag, the status reverted to "Bound to Account." Note that Ethereal Keys are also "Bound to Account," not "Bound to Character."
I wonder if the unique status of the Ethereal Keys as account bound collectibles is causing the problem, and the game engine is just checking the "bound" status of items within the bags. The only other non-equipment bound items I know of are ingredients, though they are "Bound to Character," and they also temporarily change the bound status of the bag to "Bound to Character."
So, if your non-DDOstore bought collectible bag's status seems to have randomly changed, check if you have any Ethereal Keys inside, or other bound collectibles. Remove them and the status should change back.