This could be some new (and rude) behavior from the trusty Echo, but I am thinking it is a variation of some of the Bluetooth issues some of us have experienced, where a BT device somewhere in the home reconnected, and/or it could have been created by someone driving by playing that music on some device in their car, (and it has a defect in it's electronics) and it was spewing out RFI, better known as Radio Frequency Interference; something akin to one radio channel bleeding into another.
I have actually had this happen to a Bluetooth speaker where it was on, but not connected to any device of mine. It was picking something up momentarily from some passing by auto.
We have had a few instances where someone (me included) had forgotten to turn a computer Bluetooth off that was connected to the Echo, and a few hours later Alexa blurted out something. Many of us regulars here are now more careful of leaving BT on accidentally anymore.