@Emily Horton Congrats on your decision to move to Westchester County. I can't say I am surprised that you chose this area since the Real Estate has blown up over the past 2 years in both Yonkers & Mount Vernon. Investors came in with a full blown force and made the value of properties go up.
The one major difference is Mount Vernon you have the taxes eat away at most of the profit you get and any multi family higher than a duplex is most of the time not profitable.
Yonkers taxes are extremely reasonable and lots of value add in zip code 10701.
Now, to answer your questions:
-Is there a reason that a bunch of multifamily properties have appeared on the market recently?
Since I am an agent and investor in both areas I speak to investors all day. People sell their properties and trade them for others all day and I have not come across 2 investors yet that are selling for the same reason. "Each to their own" they say.
There is absolutely NO REASON that anyone can give since its a question thats to broad. And if someone is telling you a reason they're delusional.
-What is the level of demand for multifamily living in Westchester County?
The Demand is so hot right now and the market is so tight. Speak to any agent in the area and they will tell you the same. Most of the deals are done "off market" and like they say"the good ones are taken, the rest are handicapped".
Hope this helps. If you do plan on house hacking you can easily live rent free and make some money to. i wish you the best of luck!