There are water currents running from east to west along the shoreline even though the main flow of water in Lake Ontario is from west to east with Great Lakes water heading for the St. Lawrence and the sea. The original sand bars that later formed the Toronto Islands came from erosion on the Scarborough Bluffs by the same process. Those currents bring overflows of storm water mixed with raw sewage from the Humber River following heavy rains which collects in fairly contained areas like First Beach where it stews in the hot sun.. The other beaches are able to clear faster. Before amalgamation, the old Borough of Etobicoke used to use a special boat equipped with rotating spiked drums to gather the floating guck and take it away in trucks.
By the way, if you closely at the "sand" on First Beach that makes it look like a tropical beach, you'll notice that it is really made up of crushed zebra mussel shells that arrived the same way.