The wicks function as a pressure seal of sorts. When you lay one on it's side (and there are a few other variables involved), you may be exposing one wick of the tank to the positive or atmospheric pressure in the center tube. That pressure transfers, and reduces or eliminates the vacuum (negative pressure) in the tank.
When you loose that vacuum, you loose one of the key elements helping to keep
juice in the tank, and out of your coil head and the air tubes above and below it.
Here's a better, although more lengthy
explanation.