BBBW: after power down, reboots automatically

I have a BBBW that won’t stay powered off. When shutdown, it powers off briefly, then powers right back on. This is consistent whether power is provided by the USB or power jack. It happens when powered off by the power button, halt, or /sbin/shutdown -h.

There are no capes attached: it’s a stock board.

Can anyone suggest a line of investigation for finding the problem?