Hi @sue2 and welcome to the forum!
Asana is just in the process of rolling out an enhancement where dependent dates will auto-shift if their dates conflict (overlap) with the changed date on the precedent task you updated. They actually had rolled out that feature but there was an issue with it, so they’ve temporarily rolled it back and are fixing the issue. You can read more here:
(Also FYI, if you’d prefer to have all dependent tasks auto-shift, as opposed to just those which are in date conflict, that capability is available in my Flowsana integration. 
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