When you sustain an injury in which part of you (whether flesh, muscle, or bone) is crushed without the nearby skin breaking (this could involve impact damage, dangerous compression, or over-rotation), blood leaks from burst small blood vessels but has nowhere to escape. This blood will gather near the surface of the skin, causing notable discolouration.
It bears noting that the leaked blood isn’t responsible for the pain of a bruise. Instead, the level of discolouration merely reflects the severity of the damage. The more discoloured the bruise, the more blood vessels have burst, and the more severely the injury has affected you.
In rare cases, people can develop unexplained bruising. This is most common in people with issues that prevent their blood from clotting normally, leading to it leaking from blood vessels. When spontaneous bruising isn’t the product of a bleeding disorder, though (or certain medical treatments such as blood thinners), it can be an indication of something more serious, such as leukaemia or a liver disease.
When bruising occurs due to a clear injury, it isn’t something to worry about. When it stems from vascular disorders or other known medical conditions, though, it can merit investigation — and when easy bruising becomes an issue for no clear reason, you should see your healthcare provider.