corky fiber , what is the cause?

[wally mcgee]

I just picked several softball sized German Pink tomatoes from my hoop house, which we used for our first blt s of the summer.
They were fantastic tasting. Sweet tomato rich flavor, but my wife complained that she had to cut out a large corky fibrous core out of them. It did waste a portion of the tomatoes.

What causes this? Is there something I can do?

Even with the corky parts, it was far better tasting than the f1 hoop house variety I grew last year.