Announcing before the game arrives makes most of the day one purchasers and fans to buy this season pass as well, so the developers could use this money to make their season pass content without worries and risk free to some extent which is paid in advance.
The first week of a game being released is usually the best selling week for the game. Because they are a month out they are ramping up their adverts for said game. It's overall cheaper to advertise the season pass WHILE people are excited about the game. It also makes it so people will know there is more coming at a "nominal" fee. Which will keep people from selling off the game right away.
Look at Bloodborne, I want DLC but I don't know if any is coming. Should I keep the game and hope or sell it off right away? (I ALWAYS sell my games). I wish I knew if there was more coming.
Games have to be blockbusters these day's. We all know that. They are trying their best to stay afloat and make as much money as possible (successful businesses all subscribe to this time old tradition). However they are trying to do right by the consumer and offer FREE DLC to people while also offering additional content (Large Expansions) at a reasonable fee.
$25 for approx 30 hours of additional content is a great value to me. You yourselves will have to decide if it is for you. Regardless, I would always suggest that you don't pre-order DLC.