It's not really that hard to find out if you want to:
James Craig submitted 10,192 valid signatures (15,000 are needed). An additional 11,113 signatures were invalid, including 9,879 that were allegedly fraudulently collected by 18 paid circulators.
Perry Johnson turned in 13,800 valid signatures. An additional 9,393 signatures were invalid including 6,983 that were fraudulent.
The Michigan Election bureau also uncovered more than 42,000 bogus signatures that were collected for Donna Brandenburg, Michael Brown and Michael Markey.
I think 'busted' just means caught in this context.
https://www.pbs.org/newshour/politic...ualifying-them