A Boomerang Pom is normally an English, or British person who migrates, normally toAustralia, but finds that they don’t like Australia after all, or it just doesn’t suit them, and they then return home to the UK.
The boomerang bit, is when they then realise that Australia was right for them after all, and they return back to Australia.
ie: A boomerang, when thrown correctly will return to the sender.
A boomerang, being Australian, is assumed to be thrown in Australia, so the person, after returning to the UK, will boomerang back to Australia.
Sometimes also called a Ping Pong Pom, although that term, in my opinion, should really be used for people who go back and forwards a few times.
Boomerang Pom would be used more appropriately for people going to Australia, but Ping Pong Pom could easily apply for any country of migration.