Because they may find the possible criminal to be you.
That is a particularly notable risk if there is a "self-defense" incident in which you are somehow involved. But even if you have no involvement at all, there may be civil or criminal negligence liabilities documented on that video which can put you at risk.
Don't just turn over videos to the police without viewing them and talking to your lawyer first.
"Show me the man, and I will show you the crime"
(Attributed to Lavrenrtiy Beria, Chief of Secret Police under Josef Stalin)
Goof point.