Under the Steam Distribution Agreement you have provided a warranty to Valve that your DLC will conform in all respects to the descriptions provided to customers, including promised release dates.
Even if there has not yet been a customer complaint, [[And I'm pretty sure there have been plenty]] Valve may take action if you delay a DLC more than a quarter (3 months) beyond the release window you originally promised.
Actions include removing the Season Pass from sale on Steam, refunding all customers who purchased the Season Pass for the value of unreleased DLC, or allowing customers the option of refunding the entire Season Pass. For the purposes of refunds, the value of unreleased DLC will be decided in discussion with Valve.
Valve may also take action if your Season Pass content is not released after one year has passed. Actions may include refunds to customers and removing the Season Pass from sale on Steam.