Just posted a response to ElGato answering similar thoughts. See post #40.
Sounds like you too are intrigued by the fact that they entered on the sub's day...
I noticed a little mention in this article that they had apparently admitted to planning it "the night before." I think it was definitely planned, but I doubt that they just started the night before. Unless they were just lucky in hitting a sub (which is a possibility), they had been watching the busses or had somebody on the inside.