Assuming that the OP has a reasonably standard employment agreement, he may well be breaking it by contacting their investors (there is the tort). Assuming that he doesn't want the job back, why would he contact them other than to disrupt the relationship between YC and his former employer? IANAL, but the basic elements seem like they are there and I wouldn't even consider doing something like this without getting legal advice first.

"why would he contact them other than to disrupt the relationship between YC and his former employer"

He might want YC to instruct other YC-backed companies to avoid this behavior in future, sparing other people this problem.