Examples of good timing that I have seen in startups:

- Rise of a new use case (e.g. Checkr doing background checks for fast-growing on-demand companies like Uber and Instacart).

- Legal/regulatory change (e.g. a key patent is about to expire, or a new govt regulation is about to impose more requirements on an industry).

- Technology is finally adequate for doing something useful (e.g. VHS cassettes were replaced by DVDs, which were much cheaper for a company like Netflix to mail. Or maybe some sensor is finally miniaturized enough to be embedded into a constrained space for a good use case).

There's still a lot of luck, but many smart founders think hard about the "why now?" question. If the answer is "this couldn't be done until the last year or two because X, Y, and Z" then that's promising; if the answer is "no one has thought of this before" or "many people have tried this but failed," then that's often a sign of danger.