A friend of mine had the same thing. No masters and got the invite - he was all civilian. He did have a good GPA and finished the degree at 4 years, but those were obviously not the only quals.

It's a crap shoot figuring out what triggers the invite. There are some obvious factors that must be met, but it seems to be very left to figure out. I asked him if he did something different right before he got the invite to maybe help. He stated nothing, accept applying to any and every place from being tired of waiting on delta - maybe that is the trick !