1) The rubble covers a trapdoor. The rubble must be removed or moved about at some moderate effort (strength checks perhaps apply) in order to make the DC of a search check to discover the trapdoor low enough to have a reasonable chance of success.
2) The rubble actually conceals a feature by completely filling it up. For example, there might be a staircase or exit in the room which is indetectable without excavation. A considerable effort is required to find this secret area.
3) The rubble is the nest of some small but dangerous creature.
4) The rubble is in fact unstable, and a minature avalance will ensue if a character crossing the room fails a balance check.
5) The trapdoor concealed by the rubble is actually large and made of heavy wooden beams and covers an oubellete and dungeons below. It's presence might be hinted at by the presence of a capstan and chains dangling in the room. The old trapdoor is under considerable strain, and if more than two medium sized characters stand on its 10'x10' area, it will collapse, sending them down the pit in a shower of heavy rocks.
6) Something valuable is buried in the rubble.
7) The presence of the rubble is a clue that the room itself is unstable and may collapse at any time. Failing a balance check while crossing the room or digging (or otherwise causing vibrations and shifting of the rubble) in this case brings the roof down on the players, and buries anyone so unfortunate to be caught in the collapse.
8) Some of the rubble isn't rubble at all, but an illusion covering a deeper entrance to the dungeon. Similarly, entering the room triggers the programmed illusion of a collapse to deter visitors.
9) All these rocks make nice ammunition for some strong rock throwing creature. This can either be smaller creatures located in the rafters or on balconies above, or a slumbering stone giant in a far corner of the room.
10) These rocks mostly aren't rocks at all, but the resting form of some elemental stone creature.
11) The rubble can be assembled into something that provides a clue.
12) The rubble can be assembled into something that someone had a very good reason to turn into rubble. Reassembling the broken peices creates something very dangerous.
13) The current floor of the room is actually the 2nd (or higher) floor of the original multilevel chamber. Some of the spaces between the stones are big enough to squeeze down into, where they form a small 3D mazel difficult to traverse roughly defined chambers (climb checks required, failure indicates 1d6 damage). After a series of climb checks, with a successful Escape Artist check, the players can eventual worm there way down to the bottom of the rubble pile and find a corridor leading out of the bottom of the chamber.
14) The rubble pile appears to file the room to a height of 50', but with some moderate climbing and a search check, a gap at the top can be found and successfully squeezed through to reveal the chamber contuining on the other side.
