I'd like to add to @DL's answer:
Skipping further ahead in time, it wasn't a reroute out of a bottleneck, rather an introduced time delay. Either TBFM (Time Based Flow Management), or sector capacity constraints (though it also happened for northbound traffic).
The FAA website confirms Miami (MIA) as one of the airports with TBFM deployed.
The military activity off the coast is unrelated, as that area is a warning area almost year-round, with no commercial activity going through.
The following day during rush hour there was this:
The way it's always happening near Virginia Beach, hints at the metering location for the TBFM being there.