I used to have a huge lump like that at the top of my back when I was doing a lot of high-bar squats.
I have no idea what it was. It was far too large to be a lipoma, and it really seemed (and felt) like the tissue had actually just thickened up underneath the skin as a response to bar--sort of like an internal "callus".
I've seen it in about a dozen lifters through the years.
Mine went gradually away as I started positioning the bar lower on my back.