Why would someone exercise an option prior to expiration rather than sell it?

Title, is there a good reason to do this? Someone exercised a put I had sold yesterday that expires today… but why? Wouldn’t they have been better to have just sold the put and sold the stock on the open market?

Or maybe the market messed up briefly and they saw a quick arbitrage play where the price for the stock + put < strike ?

The only other reason I can think is if they are able to assign after hours? The underlying stock was moving up after hours so maybe in response to that but I thought options were strictly reg session..

That’s the only two reasons that make any sense to me but I’m here to learn.