It's more common that you might think.

I build these IVR's for a living and over the past few years I have seen a push for things like this more and more - whereas in years past we would let callers hold.

In this case - just a guess - but the calls in their queue reach a threshold where they will not be able to answer them all before they "close" so they drop them.

They let you IN the IVR with the hopes you will "self service" and not need an agent - so you end up with what you experienced.