My adolescent birds go through a phase of wanting to stay out late much like human teenagers! Perhaps the door is closing a little too early and shutting them out before they are ready to go in. Once this has happened a couple of times and they have found an alternative roost outside, that becomes their habitual roost. Another possibility is that the coop is too small for them all now that they are more grown up and the lower pecking order birds are feeling too intimidated to go in.
Do you have any cockerels or all pullets? An old rooster will usually round them up and get them in but a young cockerel can intimidate them and cause them to stay out until he is up roosting out of the way by which time the door may have shut.
They don't eat of drink during darkness, so your water being outside shouldn't be a problem although it's safer to have a source of water inside too in case your automatic door fails to let them out in the morning, particularly if you have feed inside.