Hi,
Well, what happened with: innocent until proven guilty ;). Because what could have happened here:
- Open Site Explorer found more links in their niche/industry so they just found more links in there area.
- They have started a partnership with a site that is linking to them from hundreds of thousands of pages. Have been there done that and then usually this happens. It's Links, not referring root domains what we're looking at in the end.
Some ideas to identify if this is really the more black hat kind of stuff, identify patterns:
- Look at their changes in follow/nofollow links so you get some sense if there links are bought or not.
- Look at what kind of anchor texts they're using. If the majority of links is still with their brand name anchor text then they might have done link buying or whatever crappy tactic but at least not focused on a certain keyword. If they're dumb they did it for specific keywords they want to rank for.
- Look at other competitors in their industry, do they see a similar trend.
- Have they done any other outreach/PR stunts in the past that suddenly got them tons of new links?
All of this might give you a better sense into if this is shitty black hat or that it's a valid tactic.