Probably the cooler water temps of winter/early spring along with the annual migration of pelagic species (king mackeral, cobia, tuna, etc) up the Florida coast. On the ocearch tracker, you can click on one of the fish icons and it will show the migration route of that shark. Most are around the tip of Florida and into the gulf during the winter/early spring then a water Temps start rising they make their way back up the easy coast where the water is cooler. They've probably always been in the gulf but with the increase in technology (better tracking systems and reporting) sightings, catches, etc have risen.