Han Solo was the first person to be frozen in carbon di

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Perhaps Boba was like, "Damn - this works pretty well". Thus a new methodology was born for subsequent bounty hunters.

In the game "Star Wars the Old Republic," freezing bounties in carbonite was the norm. In fact, 3,700 years before episode I, bounty hunters would actually carry carbonite spray guns attached to their gauntlets to freeze the quarry the moment they found them. Carbonite also is good at putting out fires.

Note: the most bizarre carbonite victim I ever saw in that game was a Hutt...of all things, frozen in carbonite. I mean, it's one thing to freeze a bipedal humanoid in a carbonite slab, but a Hut is so huge, it would require an entire cube of carbonite!

Han was hanging in Jabba's palace for quite a while, and lots of bounty hunters saw him. It makes sense they'd think Boba had done it on purpose (why would he bother explaining Han was only frozen because Vader was seeing if humans would survive the process?) and start doing it themselves.

I doubt that. Why would Vader assume it would be fine to freeze Luke in carbonite, someone he definitely didn't want to kill or otherwise harm, if it had never been done before? Why would C-3PO, the worry wart, be so casual and confident about Han's safety once he found out he'd been frozen in carbonite? When Leia freed him, how could she have known that he had "hibernation sickness" and that his eyesight would return in time?

Vader didn't think there would be any problem freezing Luke in carbonite until Lando warned him of the danger, and this is the dialog:

Vader: "This facility is crude, but it should be adequate to freeze Skywalker for his journey to the emperor."

Lando: "Lord Vader, we only use this facility for carbon freezing. You put him in there it might kill him."

It sounds to me like it was a common practice, but the concern was the crudeness of that particular facility, i.e., it was made for use on inanimate objects rather than on sentient beings. I would expect a facility intended for freezing sentient beings would have a doctor or medical droid on hand, plus a more refined freezing process (such as a specific rate of freezing optimized for whatever species is being frozen), along with various computer-controlled safeguards in place monitoring the process to make it as safe as possible.

I know it sounds weird, but this was a common thing for bounty hunters to do in the Old Republic Era, thousands of years before Lucas's Anthology took place. In fact, bounty hunters would even carry wrist-mounted carbonite sprayers to freeze and preserve their quarry right there when they caught them. So in fact, it was a practice whose knowledge had been lost for millennia before the Ugnauts at Cloud City discovered it again.

If you play the Bounty Hunter storyline on "Star Wars the Old Republic," (which takes place 3,700 years before Episode I), you get to use carbonite frequently when capturing targets. A stranger, but more interesting way to use carbonite, was to put out fires.

Obviously it became a thing after others saw how well it worked on Solo. This is a non-issue.