If "blood thinners" are actually anticoagulants why can they cause easy under-the-skin bleeding with or without a bump or injury? more = worse for me

Lack of clotting: There three mechanisms to control bleeding into the tissue from injury, 1. contraction of blood vessels, 2 Platelet aggregation to form a plug at the site of injury to the vessel, 3. Blood clot formation. If any one these is less than normal bleeding occurs in the tissue. Anti-coagulants impair blood clotting and a side effect is bleeding in the tissue, or easy bruising.