Speaking in general terms, the cause of this in any type of equipment used in cars is simply because the "memory" power lead from the unit has been joined onto the "live only when ignition on" source rather than where it should be on the "permanently live" supply. It should be appreciated that all devices where a clock etc is involved "always" has two power input leads, that is a red for the ignition switched supply and usually (but not exclusively) a yellow one for the memory, some people make the mistake of getting them around the wrong way or just joining the two together, which you dont!