Why did Dave Ramsey's loans get called?

[Jack B.]

Considering that the loan documents state that my loans can be called any time (I think it was 90 days? It's been a while since I signed one but it always stands out in my mind when I do) every time I buy another property it stands out to me in my mind. Right now I'm a few months away from being able to pay off my properties. And here I am about to buy more to take advantage of low interest rates. It does make me nervous...

On one hand I want to keep the 2 mil I have, on the other I want to grow it more/faster.

So that begs the question, why DID Dave Ramsey's loans get called and how common is this?