Scientifically, what causes fat to burn? (for instance maybe heat, sweating, movement, a certain chemical in your body?)

Fat doesn't burn: Not in the way that you are probably thinking. The triglyceride molecules (TGs) are pulled from your fat cells and sent back into your circulation. Your muscle cells can burn the fatty acid contained within the TGs. Or return to your liver. It has this amazing ability to convert those TGs back into glucose. But ... you have to burn up the glucose (glycogen) in your muscle cells & liver first.