Adding a new variable to a regression model makes other independent variable insignificant

Hello, as a robustness check for my regression result i add a new variable to my multiple regression model explaining abnormal performance around CEO turnovers. The new variable is a dummy, which equals 1 if the CEO depart involuntary and during a period of crisis. However after adding it, my initial dummy, which shows the effect of an involuntary departure is no longer significant. What means that for me? Is it due to multicolinearity between the two dummies? Can i say than my results are robust ( all other independent variables have the same sign and significance levels) or not?

Can you help me?
Thanks!!!