Who crews these things? Who has had sufficient technical training, maintained currency, and descended low enough in their personal life to consider this as a way to live?
Hmm, I would suggest doing some reading on the Chicago Gangs in the 1920s and 1930s for a starter.
You might also take a look at some of the South American revolutions in the later 1800s, where control of the navy was a major factor for the revolutionaries, and some of the vessels were borderline pirates. Although the Peruvian ironclad Huascar was declared a pirate. Being on the loosing end of a revolution would do it very quickly.
Real pirates today come from impoverished backgrounds, and can achieve a weapons overmatch against their adversaries, in an environment with sufficiently little security, to make the actions worth their while.
See above, but also consider the potential for criminal activity when tracking someone down over several planets is not going to be easy or quick. No faster-that-light communication is a big headache for law enforcement. As for impoverished backgrounds, are you saying that all criminals come from impoverished backgrounds?
So who becomes a pirate? For how long do they do it? Are corsair vessels grimy things manned by disillusioned spacers who are almost slaves to their captain and his core crew who control the ship security system? Or are they the morally bankrupt who are hoping for a quick capture & sale of a vessel in order to get a share that would enable them to travel to another sector and live out their life in style and ease?
I would recommend doing some reading about historical pirates, particularly the articles covered in Alexandre Exquemelin's book
The Buccaneers of America, where they clearly were not slaves, and had a set compensation list for injuries, among other things.
As for who becomes a pirate, in the OTU you have Vargr, Aslan, and Sword Worlders to start with. Add disaffected Solomani eager to twist the tail of the Imperium, naval vessels on the wrong side of a revolution, Belters looking for some quick returns, and frontier planets desperate for goods at cheap prices, and "Voila", you have pirates. Not a moral bankrupt, except maybe the Belters, in the lot. Then you could have revolutionaries out to seize a ship to transport or acquire needed weaponry for their activities. And that is just in the OTU.
Not sure about the K'kree, although given their extreme hatred of any meat-eater, they could probably justify piracy of human/Vargr/Aslan ships as simply vermin control and a positive public good.
If you have an alternate Traveller-based universe, where the Imperium either does not exist, or is much smaller and weaker, the possibilities are pretty much endless. Consider A. Bertram Chandler's Galactic Rim, where odd things slip in from other universes, such as the oversized intelligent Rats in
Contraband from Otherspace.