Improper handling of the catheters contaminates them and they become conduits for bacteria from the outside to the inside, thereby bypassing the host defenses such as skin and mucous membranes.
The catheters cause injury to the mucous membrane of the urethra, making the tissue very susceptible to inflammation.
The insertion of the catheter produces small breaks in blood vessels lining the urethral membrane, allowing bacteria from the blood to enter the urethra and eventually the urinary bladder.
The catheters are not sterilely packaged. Bacteria on the catheters then ascend up the urethra to the bladder.