Ask HN: How a DNS problem can be limited to a geographic region?

Some of my co-workers working from France and Germany (I'm in Switzerland) started to have DNS problem with slack a while ago.

Apparently, slack is investigating the problem (https://status.slack.com/2016-09/5c66a4f2b732d3df I hope it's a permalink).

This really puzzle me, how can a DNS "outage" be limited to EU and middle-east? Why would Switzerland not be affected? It seems to me we are in Europe when it comes to internet connectivity. Either 4G nor my own DNS server had the issue here.