I've seen old (locked) posts about this, but it's only started happening for me.
I put my Edge 1040 Solar on a charger at the end of each ride. Once on the charger I'll turn the device off so it's just showing the charging screen with how full the battery is.
This is what I've also done previously with my 820, 1000 and 1030.
In all cases, once the device was fully charged it would turn off. My 1040 also did this, until recently. I suspect due to a firmware update, now it turns on. This is very annoying and wastes power.
I should note that I can't confirm it turns on once full. All I know is that previously I'd plug it into a charger at night, And when I went to use it again in the morning it would be fully charged and switched off. Now, it's fully charged and switched on. I don't know if it turns on once full, or after some amount of time on charging (as was mentioned in old forums).