Why would someone sell a 100% leased duplex that has Positive CF?

[Cullen Hardy]

I've always wondered this but why would someone sell a duplex or even a single family home when it is leased with long term tenants and the cash flow is positive?

I know maybe some people need the money now but is that the main reason? I'm always suspicious of this and wondering if maybe I shouldn't be. 

New here so thanks everyone!