If I had to guess, it's because I interviewed with them when I was still in college and didn't get the job, but remained on their radar for later follow-up. This ended up being seven years later though, so who knows.

Another guess is that I had a recently listed promotion to Lead Developer on my LinkedIn account. Anecdotally I've heard similar experience from other people who started getting more high quality contacts once their job title on LinkedIn reflected a more senior role. I know we all hate LinkedIn but it matters because people on the hire side use it extensively, so spend some time entering details in there so that it's as impressive as your resume.