It can be probates, tired landlords, vacation homes, foreclosures, tax properties, etc. 

What happens is, people come down on vacation for a few weeks, decide they love it so much and want to buy a place, ask the Realtors to show them tons of properties, then you never hear from them again.

vacations, excuses, holidays, illness...

Things like: business or real estate seminars or classes, business or finance or real estate books, travel/vacation expenses to go look at real estate, meals when I talk about business, etc.

The Seller made a mistake but you said they moved in "without a lease" so you should have stuck to your guns and given them 30 days notice to vacate.2.

You will need someone to be able to check to see if units are cleaned out after vacating, overlock/unlock delinquent units (assuming you don’t have more advanced door technology), and to deal with units that need to be auctioned.

So my tenant vacated the end of September and they were a great tenant.

Guests who are used to book holiday homes know quite well they have to plan for their vacation weeks if not months in advance.

You could also look at giving them rights to come out to the home for one month out of the year - a free vacation home if you will. 

In my apartment complex I received a notice I was out of compliance with my renter's insurance and I had 3 days to vacate or provide proof.