As soon as the angels sinned, they were not in Heaven anymore. Heaven is being in the immediate presence of God. You can’t bear the presence of God’s overpowering love if you aren’t full of love. You kick yourself out. I think there’s one school of thought that St. Michael and the other angels were mostly fighting outside of Heaven with already fallen angels, but there are other explanations too.
The problem is that we really can’t understand fully what happens with pure angelic spirits living in eternity, although there are logical conclusions that can be drawn from Scripture and the magisterium.
I’m not really sure who the good authorities are on this stuff, either.
Apparently St. Augustine and St. Fulgentius of Ruspe were of opinion that, when the bad angels chose to be prideful and to take an aversion to God’s goodness, they ran off from Heaven of their own free will and then started doing bad things. But they fell at the very moment when they chose not to love God, because spirits are more permanent in their mental decisions than humans can ever be.
