Locally, BGP is peer-to-peer — literally! — and no particular peer is forced to check everything, and nobody's even trying to make a single global routing table so local agreements can override anything at a higher level.
A route leak is often like this: an ISP in Pakistan is ordered to censor YouTube, so they add a route internally to YouTube's IP addresses that passes to their censoring machine, or to nowhere. They accidentally have their edge routers configured to pass this route to all their connected networks instead of keeping it internally to themselves. Some of their peers recognize this as the shortest route to YouTube and install it into their own networks. Others recognize it's not the real YouTube and ignore it. Transit providers check route authorization more thoroughly than peers, so none of them accept it and the route doesn't spread globally.
An isp have lease a new 10Gb fiber to youtube for my own customers, the route is leaked to my peer and now every isp in the whole country is using my fiber for youtube.
Locally, BGP is peer-to-peer — literally! — and no particular peer is forced to check everything, and nobody's even trying to make a single global routing table so local agreements can override anything at a higher level.