This can be due to two things. Either too much oil was used when seasoning the griddle, or you turned off the heat too early and didn't let the oil burn off completely. Fortunately, there's an easy fix. Turn the griddle on to high heat and let it heat up for 10–15 minutes. Scrape off as much of the thickened, partially cooked oil as possible. Then re-season with a very thin layer of oil.
Here's a video explaining how to fix an over-seasoned griddle surface.