Because he was about to renege on the previously agreed EU-Ukraine agreement and hand over Ukraine to Russian control. A little fact that Russian apologists conveniently always ignore. It is also a fact that was removed by vote of the democratically elected parliament and that a new election for the president was immediately called. The Russian occupied areas could have participated in this and the following election if it were not for the Russian occupation.
So violent unconstitutional Revolutions are ok if a President makes unpopular decisions?