It's relatively well known not to shower with contacts in since it can cause the microorganisms in the water to get into your eyes causing permanent damage. These microorganisms are however exclusively dangerous with contacts, since the eye itself is really good at flushing away anything unwanted every time you blink.
With the contacts in place, the water has a place to get trapped and stick around. Water can also cause contacts to swell leading to normal usage causing micro scratches that lets bacteria and amoeba get past your eye's natural defenses.