@Alex's Silly Question
I am not the pathologist whose comment Alex requested but....
To have a heart attack in these circumstances, you have to be predisposed to heart attack (otherwise everyone struck by a police baton and pushed to the ground would have a heart attack).
Heart attacks are caused by blockages in blood vessels around the heart. (Predisposition to heart attack usually means that those vessels are constricted and are therefore more vulnerable to blockage by blood clots than vessels in good condition.)
I can speculate on two contributions to heart attack in this case:
Firstly, stress contributes to heart attacks (apparently by increasing blood pressure - a process in which the body "deliberately" constricts blood vessels. Where blood vessels are already constricted, recovery from stress tends to take longer than it would do in someone in "normal" physical condition). I would contend that being attacked by the police was a stressful experience.
Secondly, blood clots are formed by platelets which can form in response to damage to blood vessels - clotting is part of the process of repair to damaged caused by contusion - for example, being struck by a baton. So where there is bruising, there is increased formation of clots. It may be possible that some of the clotting agents are swept along in the blood stream to areas where they are not needed - and in the case of already constricted blood vessels, to places where they do damage. Clotting is not an instant process (otherwise all wounds would heal instantly.)
So, I can imagine a chain of events from assault to heart attack.
For a more informed view, a pathologist and a coroner are required.