A lot of connector series are are multi-sourced because big clients tend to require this. For example the 38999 series connectors used in military and aviation applications are made be TE, Amphenol, Souriau, ITT Cannon, Eaton...
So it's really not uncommon to have manufacturers make something thing that a different company is known for. I think it's basically just luck that Molex got the credit for it
38999 is a bad example, because the standard came first. Better example might be something like LEMO F-series connectors, which might just be called LEMO connectors in some contexts, yet have compatible connector series from at least a couple different vendors.
There were similar connector standards that preceded 38999, e.g. MIL-DTL-5015 as far back as the 30's, which evolved into a couple of subsequent standards, with 38999 emerging in the 70's.
Edit: just realized you were maybe referring to the LEMO F-Series compatible connectors. Sometimes a company just designs a new connector. The F-series stuff gets used on stuff like instrumentation that may or may not be ingress protected, and has a need for extremely small size and potentially getting mated/unmated often in service. My assumption is that LEMO found that there wasn't a connector on the market that did this well, and spun up a niche.
So it's really not uncommon to have manufacturers make something thing that a different company is known for. I think it's basically just luck that Molex got the credit for it