(Just my head cannon!)

Before Star Fox Assault, Pigma was at the Star Wolf base, and was attacked by some purple smoke. What happened next is that he lost control of his consious, and the true villain emerges from the smoke! That villain was Zorak, a Krampus-like creature, that feeds off souls, to gain power! Ever since Wrenchy defeated him, he was looking for a vessel. But now, Pigma betrayed Star Wolf....

Zorak, Wrenchy © Me

Pigma © Nintendo