Why would people get kicked out of a duplex Home, or have to move out?

Lease expiration, lease violation, sale of property subject to vacant possession. Health department or habitability issues, condemnation... There's a long list of possibilities.

Most of the time, they have violated their lease. On a rare occasion, the owner has to move in himself. For example, his investments went to H*ll and his other two homes were foreclosed on (one of which he lived in), leaving him with no other place to live. By law, he is entitled to evict and move in himself, or just ask them to leave, without an eviction, which would be better all around.

Property sold or foreclosed on ,tenants breaking the rules of the lease, non payment of rent, damage to property