Because they are trying to grow towards the surface where they will hopefully find light. Almost all of us that have done several day blackouts have the same WOW! reaction when we take off the covers. Just like in tanks with lower light than ground cover plants need where they grow towards the surface rather than across the substrate, in blackout conditions the plants feel they need to grow towards the surface to get to the light.
And if I remember correctly from a while ago, they don't actually increase in biomass (weight), they just elongate by burning stored sugar.