Question about domain controller dns issue?

I have 3 separate physical stores, each store has its own domain and domain controller but all part of same forest. Each domain controller also handle the dns for their store. Store A was the first dc, B was second doming brought into forest, and C was the third. If Store A domain controller goes down, how come my dns is affected at B and C? Websites will come up here and there but there is an obvious dns issue there. I thought that because each domain controller handled their stores dns, they would be fine if the domain controller in store A went down. Of course store A users would be in trouble with no dns server, but why are B and C having issues as well. Any help would be much appreciated, thanks???