IME, the force of ejaculation varies from time to time. It depends on a number of things and isn't really subject to conscious control, but if I am particularly turned on it is often more forceful.
Just treat it as one of those variations.
If you want to avoid getting semen on your face, make sure the penis is pointing down towards your stomach / breasts / wherever, rather than up so that a forceful ejaculation would sail in an arc over those...