: I've wondered for a long time just what caused Tycho's switch in
: motivations...
I think it was a narrative requirement. As Durandal's motives (and megalomania) expand beyond the initial stages of contacting the Pfhor and taking advantage of the resulting chaos and carnage, Tycho has to step in and be the human-hater. Durandal started out as the one who hated humanity-- resenting his menial tasks and wanting revenge on his creator. Later he finds time to be magnanimous, helping defend the colony, giving Earth weapons against the Pfhor, saving/repurposing the BoBs. There's no need or room for hatred towards humanity as Durandal refocuses on surviving the closure of the universe and becoming God.
Tycho has to oppose Durandal; so in M1, that meant avenging the human deaths caused by Durandal contacting the Pfhor. Once Durandal expands his motives, that means Tycho needs to ally himself with the Pfhor as the only force capable of resisting him.
All that would really be necessary would be an indication that Tycho is in the Jealous stage of rampancy at this point, and that the object of his jealousy is Durandal.