Sinkhole?

The rise of the al-Nusra Front and ISIS, and the growing strength of similar extremist groups with tens of thousands of fighters, raised the prospect that the defeat of Assad—or even a stand-off with him—might lead to the establishment of an Islamic state in all or part of Syria. In an interview with the New Yorker, one senior US official captured the concern when he said, “you could have a situation where the more secular rebel groups could well be fighting the more Islamist-oriented groups. I call it the Sinkhole. I think there is an appreciation, even at the highest levels, of how this is getting steadily worse. This is the discomfort you see with the president, and it’s not just the president. It’s everybody.”

What does the Sinkhole possibly mean in the paragraph?
What is the connection between this word and the previous sentence "you could have a situation where the more secular rebel groups could well be fighting the more Islamist-oriented groups"?