The phrase "mill bust" generally refers to when a large industrial mill or factory goes out of business or shuts down permanently. It means that the mill is no longer operating and all of its workers are likely to lose their jobs. Full definition
This is the third major puppy mill bust for the two organizations in less than three months. (hsi.org)
Here is a video from a 2011 puppy mill bust — it is important to be aware of how these dogs are kept and treated. (charitypaws.com)
She is an active volunteer of the National Disaster Animal Response Team (NDART) and was deployed to New Orleans following Hurricanes Katrina and Rita in 2005, to San Diego following the wildfires in 2007, and most recently to a Livingston County puppy mill bust in May 2014. (michiganpetfund.org)