What might be the possible cause for reduction of current user count from defined load in middle of steady state period

I have set up Scenario settings of 50 user for 35 mins with Ramp up period of 5 mins. After starting the run, as per the settings all the users were entered into system after 5 mins.However after 5 mins of staedy state, 7 users exited system though still 20 mins of staedy state period is there. Would like to know what could be the cause for such an unexpected behaviour