They just up and set fire to their already suffering goodwill.
It's always been the case that the value of Reddit is literally it's users... and they just took a huge step forward in trying to eliminate their "dependency" on those users with AI, which was almost certainly trained on the very posts the users made. Those users were neither asked for permission nor was any serious attempt made to inform them that their previous works were about to be used for an effort to replace them--effectively taking away their agency to keep Reddit in check by withdrawing from the site if the environment became too toxic.
This isn't really a matter of copyright, per se. It's strictly a matter of the social contract under which Reddit benefitted always had the restriction that they couldn't screw over the users because if they drove the users away, Reddit would be "done". They tried to break that social contract and keep the benefit of having active, knowledgeable and engaged users without the responsibility of providing a reasonable place for those users to meet and communicate. That's taking agency from users, which is counter to everything else Reddit represents and objectively evil, so of course their value dropped.