The water will wash more stuff into the load, so first you're reading over more bits of stuff that can cause an unplanned deflation event (UDE) (please stop mentioning the P word even by abridged reference, I’m getting nervous). Secondly, it’s more likely that the water causes the pieces of flint to stick to the tyre, rather than failing off and with each rotation it can get push further into the tyre causing the afore mentioned UDE.
The reason the piece of flint sticks to the tyre is probably something to do with the surface tension in the water layer between the piece of flint and the tyre; but, I’m sure there is somebody on here that knows the exact reason.