The authorities might have got suspicious at the amount of electricity the house was using.
I thought about that too. I remember seeing a report about 10 years ago where electricity usage (for all the grow lights) was used to ID possible pot growers. But with all the electronics and power draws in my house (lots of CRT monitors), I'd be surprised if most in-home-pot growers could be identified in that manner. Particularly since those lights all seem to be fluorescent based, which take less power than incandescent.
That being said, these guys look like they've got a huge operation going on there, so I guess it's possible.