If router A will send information to router C he takes the path over B because the metric is much better.
If now link (B, C) fails Router B makes the mistake not to send out an update, but takes the route over router A to reach router C, he found in his routing table. Problem is that router A don’t know about the broken link (B, C), so he still tries to reach C over router B.
A message from A to C is sent to B and router B sends it back to A for reaching C.
So a loop has occurred.

"Looping Problem" is mentioned on: Distributed Bellman-Ford | Highly Dynamic Destination-Sequenced Distance-Vector Routing