I don't think many subprime loans were made on small multifamily because the down payments would have been higher dollar and I don't believe subprime loans were available on apartments because that's the commercial space.
What I experienced as a property manager was that people in the construction trades were losing their jobs and needed to move out. As shadow inventory from housing increased rents had to adjusted to be competitive because apartment units were now competting with the glut of homes on the market. Some people were not paying mortgages on houses they had as rentals and collecting rents in order to have some income because they were going through financial hardships.
Apartments owners that were overleveraged and didn't operate their properties well, lost them to foreclosure. If they didn't adjust to the market and had a hard time keeping units occupied would have difficulty making mortgage payments or taxes and insurance.