Envelope analyis basic question
A high-pass filtered signal has lost its low frequencies, so you can refocus the spectrum on a higher frequency range. 
When you rectify it, the first phenomenon is that the signal becomes positive. Hence, it is not zero-mean anymore, and thus recovers low-frequencies that were attenuated in the high-pass filtered signal.

A second is the presence of higher frequency terms. Typically, when a sinusoidal component has period $T$, its absolute value doubles its period and generates higher-order harmonics.