It can seem like there’s no rhyme or reason to which server is selected. However, the way a data center is picked is by determining the lowest combined ping of everyone in the match. That alone can result in some seemingly strange selections because not every pathway through the internet is as good as the next.
And there’s also another factor that’s somewhat tied to ping: physical location. The regions have several data centers within them, so it’s not just a single NA, EU and Asia server. Using your situation as a example, this allows the possibility of players, who are all logged into the EU region, to be located closer to an Asian data center, and so will have a lower ping to Asia than EU.
Plus, ping is always changing as pathways/backbones/nodes experience traffic changes, technical issues, or lapses in maintenance, etc.
The multifactorial aspect can make it seem like it’s random selection, but it all comes down to the combined ping of all the players. And the more players there are, the more complicated the selection process gets.
Below is a simplified example that I wrote up a little while back. I’ve edited the quote to include regions so that it illustrates what you’re talking about here.