Three of the most common theories for what induces hypertrophy are mechanical tension, muscle damage and metabolic fatigue.
The prevailing theory at the moment is that mechanical tension is the main mechanism for inducing hypertrophy, with the other two possibly playing a smaller role. 
More specifically, activation and mechanical tension in high threshold motor units (type 2 fibres) induces hypertrophy, whereas activation of low threshold motor units (type 1 fibres) induces little to no hypertrophy.
The theory is that the muscle cells have a mechanism whereby they sense being physically stretched, which triggers off the hypertrophic cascade.
It seems that mechanical tension would highly correlate with activation. In that, when electrical signals from the nervous system are telling the muscle to contract, a stronger electrical signal would result in greater contraction and therefore greater mechanical tension.
My theory is that an alternative mechanism could be that the muscle cells sense high levels of electrical activation in type 2 muscle fibers, and it is this which directly triggers the hypertrophic cascade, rather than the cells sensing being physically stretched.
Has any research been conducted as to whether it is the electrical activity itself which triggers the hypertrophic cascade rather than mechanical tension?