A few pessimistic explanations for the OP, not necessarily mutually exclusive and in no particular order:
1.) Conversations were already under way about a transition to Ollie - and they decided today, given the bad news already in the air, was an opportune time to announce the add'l bad news, in order to minimize the impact...get it all on the table at once.
2.) Calhoun was hanging in their longer than he wanted, in the hopes that his continued presence would make UCONN more appealing to the ACC. When the catalyst that they were waiting for (an ND move) came and went without effect, JC threw in the cards and retired.
3.) The opposite of #2. Manual and Herbst are desperate, and asked Calhoun to retire now as a last ditch effort to appeal to the ACC. Perhaps they think ACC is passing them over because the BBall team is "scandalous", or for fear that the BBall program is a "one hit wonder"; and forcing the move now is a chance to correct these impressions sooner rather than later.
Is it getting dark in here.