signal blackspots, cheap wifi routers that get overloaded, spectrum congestion from too many things, deliberate throttling by ISPs, deliberate throttling by users when abroad to save roaming costs, VPNs, Windows networking drivers, proxy machines, unstable SSH tunnels, misconfigured caches, broken CDNs, etc
also note, the method that article used was used on theverge.com, so half the images would fail to load because ghostery would remove a tracker, then a script would fail that depended on that tracker preventing the lazyloading code from running