It would appear that it is written into the House bylaws?
Well a nkn binding legal opinion from.Paxton
https://www.texasattorneygeneral.gov/sites/default/files/opinion-files/opinion/2021/kp-0382.pdf
Summary
Texas courts recognize that a vacancy may occur by
abandonment of office. Whether a specific legislator abandoned his
or her office such that a vacancy occurred will be a fact question for
a court and is beyond the scope of an Attorney General opinion.
Through a quo warranto action, a district court may
determine that a legislator has forfeited his or her office due to
abandonment and can remove the legislator from office, thereby
creating a vacancy.