Personally, I don't think this is that big a deal. If you're a good engineer, there really is no shortage of opportunities. If you're not a good engineer, then that is the root problem that should be fixed. For me, recruiters are just people who helps me do the logistics work so that I never had to apply for a job.
"To this day, I don’t know what the recruiter said to my former manager, but I do know that my manager called me up pissed off about the phone call he’d received, and told me to not use him as a reference in the future."
I think that's a bad manager. As a manager, especially in tech, you should definitely expect your co-workers to take the best opportunity available. As a professional, the manager him/herself should also take the best opportunity available. It's people's careers, be professional about it.
Well of course, but I mean that as if you're not confident enough to recommend other engineers because of competition, then the root cause is probably your own standard of expertise that you have set that you have not met and should meet.
"To this day, I don’t know what the recruiter said to my former manager, but I do know that my manager called me up pissed off about the phone call he’d received, and told me to not use him as a reference in the future."
I think that's a bad manager. As a manager, especially in tech, you should definitely expect your co-workers to take the best opportunity available. As a professional, the manager him/herself should also take the best opportunity available. It's people's careers, be professional about it.