There are quite a few things that could cause it, the best thing would be to check it at the PSU and see if your PSU -12v is correct. Otherwise you'll have to do some tracing.
It isn't a good thing that this voltage is low and could cause some problems. Some components rely on voltages not only to run but some use them to shut off too. Some transistors switch on when a bias voltage is applied and others switch on when a bias voltage is applied. If a transistor is supposed to shut off when it is biased but too low a voltage is supplied to do this, then you could end up with a fault somewherefurther down the circuit because of too much voltage being applied. We are talking about very small voltages, but when that -12v goes through a few components it will be lowered to smaller values through some circuits.
If that is confusing, just think of it like a valve that shuts the water off when it reaches a certain level, if it doesnt shut it off, it can overflow the container.