I'm hearing and seeing the phrase "Beast Mode" a fair bit in recent years, mostly as a teen tag, pertaining to them being active or hyper-active... and a bit like saying that they are awesome.
It might have taken 10 or so years, but is this just the young kids of the late 90s Beast Wars era using a phrase in their teens from their childhood, like us old farts might say "roll out" instead of "let's go". (okay, maybe just in my head then )