My guess is when the parse tree exhausts short term memory. I have no problem reading the sentence with a single nested clause ("the cat the mouse..."), but th

My guess is when the parse tree exhausts short term memory. I have no problem reading the sentence with a single nested clause ("the cat the mouse..."), but the one with a doubly nested clause is unreadable ("the dog the cat the mouse..."). Greater nesting levels might be grammatical in an abstract sense but is extremely difficult for people to understand, much like how it's possible to come up with legal C code that results in indeterminate behavior.