In their self-important race to catch up with the rest of the species in the Commonwealth, humanity pirated an Aethan Haht Garuud explorer ship in altspace. The gambit paid off and they brought their prize home so that it could be reverse-engineered and duplicated.
Unfortunately, when the Garuud connected their miraculous leap with the sudden disappearance of one of their explorer ships retaliation was swift and brutal. It came to be known as the Retribution War. The Garuud decided that humanity didn't deserve to be a part of the interstellar community, so they would take the stars from them. Them bombed their colonies, destroyed their ships, and eventually attacked Earth, intent on destroying their altspace portal and NavCom beacon. SagCom finally stepped in, not to save humanity, but to preserve the beacon network. The Garuud were not happy about it, but they finally backed down. The offense wasn't great enough to justify going to war with the entire Commonwealth.
The devastation of the Human colonies combined with the resentment and distrust when the cause of the war came out, left a substantial portion of the surviving colonial population homeless and desperate. Earth could not supply the needs of all of the colonies directly until they rebuilt enough to become self-sufficient again. Many were so angry with the UHN that they would rather starve than accept anything further from Earth. Many of the desperate and angry survivors abandoned human space and settled anywhere they could in the Commonwealth as immigrants. Unfortunately, the Retribution War left them with an unfortunate stereotype as pirates, spies, and thieves. Many races look down on them, and humans are tolerated at best. Some, especially the Garuud think they are lower than even the Vulch.