I have no idea obviously but I wonder if its similar to whats happening when adjusting the (regular) equalizer. When adjusting a few sliders in quick succession in small steps maybe 0.5dB or 1dB, sometimes sound spikes as well for a fraction of a second.
I've experienced what you're describing, and I wondered about that myself. I've definitely had some peculiar volume transients from rapidly making fine DSP adjustment, but I've never had them blow past the volume maximum from that. The DSP updates it's effects in real time (and thank the heavens for that, I wouldn't want it any other way), so I always chalked those transients up to me typing too slowly (i.e. I deleted the existing value, and it updated the feed before I finished entering the new number).
It's odd because I can be playing music at the volume cap, hit mute and hear a 10 or 20 dB spike while the little visualizer fills up completely. Simultaneously, I can turn the volume down to 1% where it is inaudibly quiet, but if I turn it down one more percent I get the exact same incredibly loud signal and visualizer behavior.
It seems like the visualizer spike has been confirmed regardless, so for some reason JRiver is running a super hot signal somewhere in it's audio chain momentarily before the mute action is completed. It successfully suppresses that spike on NateHansen66's system (and on mine for that matter when convolution isn't running) but it's blowing through on mine when convolution is on for some reason.
Honestly, even if it is just a quirk of my setup (i.e. if the fault is on my end and not in the software) the fact that this kind of error is happening at all is causing me to seriously reevaluate my decision not to use a hardware pre-amp, especially as I'm currently getting ready to transition to some higher power amps. Up to this point JRiver's volume handling with my setup had been 100% rock solid, so I had no qualms about a direct connection, but this is making me anxious.