A 509 shows up when a server has used up its allotted bandwidth for the billing period, usually a monthly cap set by a shared or budget hosting plan, and stops serving requests until the cap resets or the plan is upgraded. Where most status codes describe something about a single request, 509 describes the state of the whole hosting account.
It's rare on sites with dedicated or scalable hosting, since capacity there tends to scale with traffic rather than hit a hard wall, which is why 509 shows up almost exclusively on cheaper shared hosting plans a site has outgrown. Seeing it at all, even briefly during a traffic spike, is usually a sign the hosting plan is no longer matched to the site's actual traffic and needs an upgrade, since every visitor who hits a 509 during that window, human or crawler, gets turned away entirely.
