From Guns.com:

When Guns.com talked to the ATF about this incident the ATF spokesman Richard Coes said he didn’t know why ATF agents suspected Myers of wrongdoing.  However, he told local media that “the claim is that he was using explosives and getting paid for it via YouTube.”

Tannerite is used in many of his videos, which is legal to own. Expect the ATF to “make a ruling” that tannerite is now illegal.