As I understand it, when closing a blade with a liner lock, say you have the blade in your left hand, you pull the locking mechanism to the left with your left thumb to release the blade and close the knife.
If you can create this same effect by twisting the cutting edge of the blade to the right, hence creating the same relative motion, this bypasses the lock and then you have a problem, as obviously this can happen at or after contact if there is any twist. I guess some locks are also easy to engage even with a normal grip which is a problem when you inadvertently pull on the lock, again if the blade twists at contact.
Some blades don't seem to have this issue, but I have heard of it as a potential hazard.
The Spyderco "Yohimbo" has a neat liner lock along the back which is cool.