the reason being that vinyl is formatted in M+S (mono/stereo) rather than LR (left/right) like a cd. the signal going to the cutter is split into mono components and stereo components, rather than left and right components. i believe the mono channel results in side to side motion, and the stereo channel results in up and down motion (but i might have it backwards). so you can visualize that as you widen the stereo of the bass, you will be rotating the wave from a horizontal to a verticle plane, which means the needle has to deal with a whole different set of forces acting on it, and sometimes it's more than they can handle.

i've got a record in my collection that doesn't play for this exact reason, btw. the needle literally jumps a quarter inch in the air, even with the weight all the way forward.