Chain coming off with internal hub gear bike - why?
In short: If chain comes off with horizontal drops (typical internal gear or single speed) the wheel has to be too far ahead. Try pulling wheel backwards until the chain can be moved &lt;5 mm and tension the wheel properly. 

Other possible issues are:


bent chainring or sprocket
wheel not installed properly
failed bearings in hub or bottom bracket


Worn chain is very unlikely. Also broken internal gears don't drop the chain.