Why are patients put on blood thinner injections before some procedures like Colonoscopies?
Isn't there an increased risk of bleeding?

I don't know why someone would get blood thinners before colonoscopy, which is a brief outpatient procedure with little or no anesthesia; but if you mean other forms of colon surgery, or surgeries in general, there is some risk of blood clot formation in the legs of a person who is immobile and asleep on an operating table for a period of time, and under many circumstances it's common to use a low dose of anticoagulant to prevent it. At low doses, it reduces the incidence of clots but doesn't significantly increase the risk of bleeding; besides which, bleeding is easier to deal with than is a major clot which breaks off and goes to the lungs. Nowadays, it's also very common to attach pressure devices to the feet which pulse regularly and increase circulation, which prevents clots without the need for anticoagulation.