I would presume that they are employed under Texas Laws, so they are in an "at will" state. There is no obligation on either side in terms of contacts of employment,
That may be true for an industry/shop where no contract is in place, but not for a Part 121 airline. The RLA and associated federal law override, and the union contract is enforceable.
Sometimes it is "too" enforceable -- hence the $40 million judgement against the APA several years ago for their "sick-out"...