http://www.huffingtonpost.com/2013/11/27/t-rex-soft-tissue-discovery_n_4349214.html
This is an article from the Huffington post. It discusses how they figured out what happened. It appears the iron in the blood preserved the tissue.
Dinosaurs’ iron-rich blood, combined with a good environment for fossilization, may explain the amazing existence of soft tissue from the Cretaceous (a period that lasted from about 65.5 million to 145.5 million years ago) and even earlier. The specimens Schweitzer works with, including skin, show evidence of excellent preservation. The bones of these various specimens are articulated, not scattered, suggesting they were buried quickly. They’re also buried in sandstone, which is porous and may wick away bacteria and reactive enzymes that would otherwise degrade the bone.