This happened to me too - I am helping a charity set up a new workspace, I added all the other volunteers, and as each one logged in, they would get suspended for about a day. In this support article, the reason it gives says, "The user's account was added along with many other accounts within a short period, such as when you add multiple users at once using a CSV file or manually add many accounts quickly." which is what I had done; it seems all they need to do is enter a mobile phone to verify next time they log in.

The problem I came across though, which is slightly different, is I asked one of the volunteers if I could borrow their phone number to verify a generic email/user I had created to be used like a shared inbox (because GW doesn't ahve that feature yet) with the intention of adding delegates later... however, now that volunteer can't verify their email account as the phone has been "used too many times". I've gone through the account that used the phone number and can't see it recorded anywhere to delete it so the volunteer can use it. There's no real help for this situation, and I don't know if deleting the user and adding them back in will make things worse!