What can cause an air lock? And why wouldnt it start?

Filled up with fuel yesterday then drove to asda, then when i came to start it up again it would start but then stopped straight away? Did this quite a few times. When i removed the fuel cap and and replaced it then tried again it started. Its not done it since. I can hear what sounds like a sucking noise dor just a second after start up. Could this be a split pipe or something and was it an air lock that was proventing it from starting or was it just a coincidence it started after removing the cap? Its a vrs cr by the way.