Apart from dengue and blood cancer, what causes a fall in platelet count (thrombocytopenia) ?

Lots of things: Certain medications, and other viral infections can do it as well. Sometimes severe illness and malnutrition can do it.

ITP and...: One of the most common is immune-mediated thrombocytopenic purpura (itp), a condition where the body makes auto-antibodies against platelets, leading to their destruction. Viral infections can suppress the bone marrows ability to make platelets. Another cause is aplastic anemia. Lastly, a large hemangioma can consume a large amount of platelets.