After researching (finding no answer) and mulling this over for a long time the only possible answer I can come up with is that Buckley may have been monitored for a long time in exactly the manner of that fateful morning and unknowingly dropped his guard momentarily because of the repetition and that was enough. It's still difficult to accept that this would have been the case but what other answer is there? As to why he would have dismissed his driver, I suppose no one will ever know the answer to that one either.