Mine started to drop automatically when I start to have issue with the window motor. First symptom was not dropping or going back up when i press the switch. Sometimes it drops, but refuse to go back up unless I manually pull it up by hand. I did try to refresh the motor by removing the housing and give it a quick clean with contact cleaner. Works OK for a while and issue starts coming up again. After 6-8 months, eventually the motor starts to get worst, dropping automatically especially when door is open. I have to manually pull the window back up to fully close before i shut the door to ensure fully sealed, but after awhile it will still drop by 0.5cm to 1cm creating a leak and I can hear it at around 60mph onwards.
Anyway, in the end, i swapped a new window motor (ebay specials) and everything is working as it should hence no more dropping automatically. Hence i guess your next step is to get a new motor installed and take it from there.