More "educated speculation" (BTW, I'm stealing that one): A flaw in the sock (or an opening in the pipe itself) is certainly one possibility. The sock will only trap larger particles. Ideally, the larger particles will trap slightly smaller ones, and those will trap still smaller ones, eventually building up a graded filter on the outside of the sock. At least that's the ivory tower thinking. If the soil is a uniform, fine-grained clay the filter sock won't do diddly to keep it out. Regardless of the cause, if soil is continuing to enter the drain system then it can eventually remove enough soil from below the foundation to cause structural movement. That would take a while, but it's a possibility. Something needs to be fixed. The question remains as to what that something is.