It's really just a colourful expression that refers to a lot of animals, like buffalo, running. It's used pretty much exclusively as a metaphor for a big trend, a lot of people going in the same direction, maybe financially, but not exclusively.

It would be helpful to have more context, but it should mean that the herd is running or even stampeding. The sound of hoof-beats can sound like thunder. So to describe the noise of the herd running, the writer uses "thundering."

It's actually a financial term, and it's difficult to understand even in English, but it literally means the sound of hooves (animal feet) in a herd, running. Within a financial context it is a term, or phrase, meaning something like too many people trying to go through a beaurocratic process all at the same time. It is difficult to explain even to other English speakers. Maybe someone with a financial background will be able to help you understand it better. The phrase is rare and difficult even for English speakers. Unless they simply mean the literal meaning, which is the sound of a running herd of animals.