Ziploc vacuumed bags expand in sous vide
Believe it or not, veggies just happen to contain a lot of air.  Even under vacuum this is a fairly common occurrence when cooking veggies low temp.  You can add oil or other liquid to the bag, it will help, but air pockets will still likely develop.  I usually add weights to the bag or bleed the air out by lifting a corner of the bag and resealing by displacing the air as it happens.