I think the reporting requirement is triggered with each additional 1% accumulated (filing requirement) - but trigger for a takeover includes other factors. I think Exelixis has a "poison pill" that kicks in if a single entity acquires 15% of the company.
It's interesting how EXEL is trading - nothing for the last few years seems to have leaked. But based on message board activity here, I'm almost certain that something will happen in the next week or so. Certain posters go quiet, others get more active. Just a gut feeling.
It's an SEC requirement to file a form when you hit X% (5% I think) of any public company. This is just a disclosure requirement - and when its a stock trading company, like Wellington, it likely just means they hold it in one or several of their larger portfolios. Sometimes, you will see a weird holding company, or in rarer cases, an actual competitor or interested third party, which could be indicative of some sort of potential M&A type activity. However, in those cases, they would typically "creep" up to the reporting threshold "4.99%" and then buy up to the max before triggering take out provisions, at a strategic time (i.e. coinciding with a hostile takeover bid). If you look back, you'll see the larger stock trading institutions files these regularly whenever they pass the "reporting" threshold. So, in other words, could be interesting, but probably no big deal.