Their system probably doesn't implement a function to delete account, or it is not easily discoverable in the UI, or it is known to be bugged. So the employee who did that thought that renaming an account was a good idea to make it appear "missing". A big numeric suffix is an obvious idea to avoid collisions with the existing and future accounts.

But when people are asked to choose a large number, numbers around 1000 and its multiples are chosen particularly often. So 999, 1000 and 1001 are very likely numbers to be picked "randomly". I don't find the reference on that, but I suppose we all have enough anecdotal evidence. Just recall what are the common port numbers of various programs. X*1000 + Y is a very common formula, where |Y| < 100.