The theory being posited is that as the comet moves in towards the Sun it will heat up, and buried volatiles will be driven off, opening up hollows.
These voids may already exist to some extent in what is known to be a highly porous body, but the lost volatiles will only exacerbate the situation.
And the dusty, rocky ceilings above the caverns will not be able to support their own weight - even in the low-gravity environment of 67P - and will eventually fall inwards.