It could be any number of reasons. However, if they banked on getting clients to pay on time and kept letting things slide and didn't know how to "be the heavy", or didn't want to suspend any clients for fear of losing future income, they could have gotten themselves so close to the edge that they leaped right over.

Working in something where you never meet your clients has caused many hosting companies to be too harsh. There's also the opposite pole, though. I have actually had business clients try to use the "well, if I move someplace else that's no more recurring income from all the clients I bring you!!!" argument. I calmly told one such client that the suspensions would go forward unless the (now-almost-30-days) invoices were paid before the approaching deadline. Said client was invited to download all her five clients' files and go find another Windows host before the accounts were suspended. Funny, but the payments came in at the eleventh hour. (This was also not the first time that late payments were made.)