If you want to unload a big drop with Zerg, you fill your overlords with units, separate them where they do not touch each other, then select all of them, and make them "hold" position. When you tell them to move they should move in sync with each other. They should all unload at once unless you tell them to unload on a specific unit/building, then they will clump together again, so you should tell them to unload by clicking on the ground or mini map. Hope this helps.
