I figured security researchers were always targets of multiple APT actors and random individuals. However...
> I've even caught them using their exploits on me after they made me an offer
Not only for exploit companies that eat their own dog food, nor only cybersecurity jobs, but I've heard of this happening to people interviewing for other tech area considered strategic.
The noticed ones weren't that subtle, and were presumably noticed because the attacker wasn't using the best methods, but maybe more routine SOP for lower-value targets.
I have no idea what the actors and motivations actually were. Speculation:
* the hiring company or its country, vetting the candidate by spying on them, including for corporate/national counterintelligence reasons (it's really not much different than a lot of the sneaky surveillance capitalism vetting that many companies quietly do, just unambiguously illegal in this case);
* the hiring company, spying to monitor the competitive offer situation (e.g., what counteroffers or concerns does the candidate have);
* other state, individual, and possibly corporate actors, for whom the imminent offer flagged the target as worth keeping an eye on (for, e.g., advance access to research they do individually, knowledge of attacks they do individually, possible technical entry point to the job-offering organization or others, or kompromat for getting access/actions); or
* random associated individuals acting on their own, recreationally enjoying the power over others that their cracking toys give them (which at least used to be not too uncommon, before cybersecurity was professionalized, when there were proportionally much more teens and alienated people, and they hadn't yet been told about color-coded hats for prefabricated codes of behavior from which they could choose; now, most people with skillz have the carrot of a lucrative job or respected status as researcher that they can pursue, instead of seeking power/status other ways and without guidelines).
Personally, I try not to work on strategic target areas, since I like to save my very limited guts for fighting product concepts and reliable systems into shape, not for being helplessly violated by lawless authoritarian institutions. Good luck.