Answer:
read explanation
Step-by-step explanation:
The Iron Warriors did a lot of the legwork for the Great Crusade, and took a lot of the dirty missions that none of the other legions would touch because they were thankless and dangerous.
The Iron Warriors didn't even have the respect of being known as the best legion in siege warfare because somehow the Imperial Fists were known for that, while many of the primarchs and legions thought of the Iron Warriors as mere grunts and glorified ditch diggers.
Basically, the Iron Warriors did a lot of the background work that made the feats of the other primarchs and legions possible and were never acknowledged for it... beyond that, were spat on and disrespected for it. That anger toward the Imperium drove both Perturabo and his legion to chaos.