Because they felt like terrifying innocent people, ogling a naked woman and grenading the cat.
“but there must have been a reason” is an attempt to justify the unjustifiable.
If there was no search warrant, then of course, this is an flagrant unconstitutional violation of rights, and heads should roll. Even if there was a warrant, the described force used was way overboard and excessive.
But the story seems very incomplete. It is presented as a completely random attack by police. Was it the wrong address? Was there a reason given?