The mini-USB power is connected to the BBB power. I can run my BBB just from the USB cable.
So you probably just paralleled 2 power supplies. The list of outcomes includes the escape
of the black magic smoke. Some regulators do not like getting started when there is already
a backward voltage from the load. The LM317 comes to mind. Its data sheet includes info
on how to make it survive. (Just as an example. it's unlikely that your laptop contains one.)
Maybe it is a good idea to use a USB port replicator during experiments with high-priced laptops.