same meaning, disassociate is just less commonly used ... probably because it is more work to say!
"dis-" indicates a reversal or a negation,
"a-" indicates in, to, towards, in the state of.
In a way disassociate has a redundant particle "-a-" in the word as all you are trying to say is "the opposite of associate" which "dissociate" does without the extra baggage!