The Bag of (Sometimes) Holding

Writing up new magic items is fun; using them is a completely different story.  This one came out of a whimsical conversation with my significant others:

The Bag of (Sometimes) Holding:  This magical item appears identical to the usual Bag of Holding.  However, it's qualities are somewhat different - while it can hold at least as much as a Bag of Holding (it is apparently possible to put in even more), the Bag of (Sometimes) Holding is uncertain in its ability to keep everything inside.  Rather than rupturing, however, the Bag will simply open and the contents will erupt into whatever area the owner of the Bag is currently occupying.  Why this happens is unclear, though some careful research would reveal that items placed willy-nilly into the Bag, or overloading the Bag, tend to exacerbate the problem.  Referees should determine a percentage chance of the Bag letting loose, based on the quantity and variety of what's inside.  Then roll 1d12 to see how many turns before the eruption takes place.  In the meantime, you may wish to provide clue of the upcoming event.

For referees: the Bag may be played as a straight-up magic item of uncertain character, but it is suggested that you allow the bag to demonstrate signs of "indigestion" - occasional loud burps, the exterior of the Bag churning every now and again, minor rumbling sounds from within.  For full effect, you can have the Bag emit a contented belch after disgorging its contents, followed by a slight sigh.  Everything inside should be perfectly fine and unhurt.  Do not let player characters use rope or other means of attempting to keep gear separated within the bag - imagine what it might feel like to the Bag!