Q:

What pressures could drive evolution of a floating creature? 

What environmental pressures could drive evolution of a creature that has the ability to float?  How it floats isn't important. 

A:

If it has multiple stages to it's lifecycle and the final stage is fairly immobile then floating "spawn" would make sense.  You'd also probably want to make it an R-strategy organism which produces many young of which only a few are expected to survive.  
 
You could also make the environment quite hostile such that food and stability in any particular area is unpredictable.  
You could also model it on jellyfish lifecycles only with floating in the air.  

Floating isn't the best way to get around but it's cheap and easy: if you're producing a thousand offspring letting them float away is a good strategy. 
Indeed a number of spider species use "ballooning" where they float away on para-shoots made of webs to reach new locations. 