What are the reasons for Nova's triple tap to reset to full cooldown? [duplicate]

Channeled abilities

Triple Tap is a channeled ability, which means that stuns will interrupt it and reset the cooldown it's full duration. You have the same effect on ETC's Moshpit or Lili's Jug for example.
You can also cancel most channeled abilities by yourself by hitting the R-button again. This is very useful if you need to escape since using such an ability make you unable to move.

Prevention

To prevent this you should identify the heroes on the other team which have access to stuns and wait for them to use it before you use your heroic. Popular examples are Thrall's Sundering or Diablo's Apocalype.

Different channel types

There is a difference between fully channeled abilites (Moshpit, Triple Tap etc.) and abilities that require a short channel before they are fired off (Hinterland Blast, Sundering, etc.).

If you get stunned out of a fully channeled ability, it counts as used, so the cooldown will reset it's full duration. But if you get stunned out of a preperation channel, the cooldown will only reset to 10 seconds.
Triple Tap belongs to both categories since it has a (very short) aiming phase before the first of the three shots is fired. If you get stunned during that, your cooldown will only reset to 10 seconds.