The Texans DID place franchise tag on Clowney.
The Texans and Clowney could not agree on a long-term deal, in part because Texans tagged him/negotiated as if he was an OLB, and Clowney considered himself a DE -- the franchise amount is about $1.5 million/season higher for DE and DE salary's are higher.
At that point, Clowney's options were a) sign the franchise tender and be under contract to Houston, or b) hold out/not play/not be paid.
Clowney learned that the Texans were considering trading him after he signed tender, so he didn't sign. That left Clowney will all of the leverage, as the Texans had a $15+ million asset they couldn't not play or trade.
With Clowney (and his "approved" teams) holding the cards, Texans got taken in the subsequent trade.
Additionally, there are reports that Clowney approached Texans about signing tender if Texans agreed not to trade him in 2019, but Texans said no. If true, that shows the level of strive between Clowney and BoB ....