Why do people sell properties that are making profit?

[Prachi Pant]

I am a first time investor looking to invest in duplexes and triplexes. I found some properties listed which are already leased out and are generating positive cash flow to the current owner. In such a situation, why would the owner want to sell a property? Is it primarily because the owner might be in need of immediate cash? Is there anything I need to watch out for?