Mulkey signing Buckner was a head-scratcher to most Big 12 fans. While I didn't think she'd ever contribute much, I sure didn't think Mulkey would show her the door after just one season. But, as has been noted, she had to make room for the 5 players she signed in the Fall. I'm guessing the late signing last summer of the Australian guard also caused numbers issues for 2015-16.
As noted in the article, however, she transferred to a Community College. That says a lot - while it doesn't define a definitive reason, she was apparently - as you and others appear to note - not ready for the Big 12 level. Throw in whatever additional factors.
The head scratcher would be the signing, but her leaving doesn't sound like a surprise. College teams are not generally charities. To be on the team, you sort of have to contribute something - even if it is just as a practice player or intangible. Katie Adams that played for Rutgers back around the mid-oughts was a leader off the court, a student of coaching, and a help for the GPA besides. Couldn't contribute much on the court, but made up for it in other ways. If Buckner wasn't doing any of this, then I would blame the coach more for taking them than ditching them.
OTH, this is why I like schools that do guarantee 4 year scholarships.