Your account is usually limited when you have received a payment from someone else who received their money illegitimately. It isn't usually random.
Never had mine limited and regularly receive 400->1K to it (bi-weekly) and no issues. Bud of mine has his rental property rent paid that way each month (> 2K) no issues. All big payments for both of us come from US verified members though (we're both US verified) so don't know how much that plays into it.
I usually move most out of it weekly for other things, but don't sweat keeping 500 to 1500 USD in it (even turned on the U.S. interest for it).
Others I know have been burned though. Guess I won't be paranoid til I am.