Trolling is a fishing technique [0] of slowly dragging a lure or baited hook from a moving boat, and is likely the meaning that online trolling takes it from rather than a creature from Scandinavian folklore [1].
There's definitely a metaphor to be made for trolling for data, that GP could have been intentionally making. I've certainly seen that idiom used before, although it could have been an eggcorn [2] for trawling.
