To my knowledge JNDI is a monitoring interface allowing you to get various system values like the number of threads, heap size, process ID, etc. Somehow it's capable of being connected to LDAP.

It sounds like a classic X-to-Y-to-Z problem. Someone connected X to Y thinking Y was pretty safe even with untrusted input, not knowing it would ever proxy to Z (probably, everything you can do with JNDI on a local machine is safe). Someone else connected Y to Z thinking that Y is only passed trusted values so the new proxy feature could only be triggered deliberately. And here we are. This class of bug should have a name but probably doesn't.

And another person didn't document well that log messages must be trusted input...