A variety of diseases. Inflammation cancer, infection and certain bone marrow diseases can all cause elevated platelets.

It depends on how high the platelet count is. A little over normal (10%) means nothing. 50% over normal means something. An elevated platelet count does not cause fatigue.
Elevated platelet counts may be caused by a bone marrow abnormality with the production of too many platelets. The elevated platelet count may also be secondary to iron deficiency anemia, infection or other rare causes.