Still not the spacecraft.
Fixed the servo board! Turns out it was user error: I had the Read/#Write line plugged into #IRQ pin on the XPAN connector - they're right next to each other. (I need a test rig for my test rig?) I guess the crosstalk was clean enough at power up to trigger the register twice.
Speaking of test rigs:
That's an Adafruit Boarduino from, like, forever ago. That and an (Adafruit) USBTinyISP programmer have been my standby prototype equipment for a long time now.