The most common reason of a leaky tank is caused by inhaling too hard or fast. By doing so, you will pull too much e-liquid into the atomizer, which will not be vapourized rapidly enough and will simply exit the tank and cause leaking. To avoid this, draw normally and steadily for 3 to 5 seconds.
The second reason your tank may be leaking is that the atomizer or coil is nearing the end of its useful life; As it ages the cotton can no longer hold in the vape liquid and you will get gradual leaking due to gravity. Make sure it is changed and primed properly before use every 7-10 days.
Lastly you may have a degraded seal on the tank. Most tanks come with spare seals that can be replaced.