How would an 'alternative DNS provider' (also known as a 'DNS provider') cause streaming video to buffer?

Let's say you live in NYC but you're using a DNS server (resolver) in Seattle. When your client performs a DNS query, you may get a response that directs you to a server/CDN/whatever in or near Seattle as opposed to one closer to you in NYC. Then, the data (streaming video) is traversing the country to get to you, passing through many more (potentially congested) interconnects/links on the way.

Reality is a bit more complicated than this simple example but such an issue is certainly not unheard of (and inspired mitigations such as RFC7871, for example).

Both Google and OpenDNS use anycast addresses for their DNS servers that are very likely to route though a 'local' server. I think ISP games are far more likely to be the root cause here...

Google DNS requests enter Google network in multiple places. DNS itself is not served from the edges.