Bottom coil clearomizers work on a pressure/vacuum system. When you fully fill the tank and screw the bottom cap back on, it creates pressure. Just like when you screw the cap onto a soda pop. As you use it throughout the day and the liquid level drops in the tank, that pressure/vacuum changes. If you let your liquid level in the tank get low (low is beneath the point where your coil and the air tube meet), it WILL gurgle, leak, or spit liquid back into your mouth.
Simple fix. Fill the tank back up again (fill it back up fully), only tighten the tank finger-tight onto the device (do NOT over-tighten or torque it on super tight), and take looooong, slow, gentle pulls. Don't rip on it hard. Let the atomizer (burner, filter, whatever you want to call it) do the work for you.
EDIT: The type of e-juice also affects coil performance. Really dark, really sweet, or really thick liquids will burn through your coils faster. Citrus flavors can do the same, also.