Eviction
Eviction is the court process for removing a tenant against their will. Nonpayment is the usual trigger, though serious lease violations qualify too. For BRRRR investors it comes up twice: with occupants who come with a distressed purchase, and with tenants in the finished rental.
Some distressed houses come with former owners or tenants still living there. Removing them takes a court order in most places, and the time it takes delays the rehab while interest on the short-term loan keeps running, $2,475 a month on the default deal.
After the rent step, eviction risk is mainly managed through screening: income, rental history and references. The process and timeline vary widely by state and city, so the vacancy budget should account for how long a turnover can take locally.
Further reading: Eviction on Wikipedia.