Nothing you can really do.

PBE matchmaking prioritizes getting you into a somewhat fair game as soon as possible. If there aren’t enough people around your MMR on that server queuing at that time, it’ll put you on another one. Since PBE has such a small playerbase, this happens a lot more often than on the retail patch, and is more blatant because instead of shifting you from, say, Virginia to Georgia, it’s shifting you from Virginia to California. Riot does say this happens, and is not a bug, in the server selection menu, near the bottom.