I’ve survived a dreaded “tank slapper” incident before, in almost the very same situation as you: bombing a hill at 40+ mph, when some unknown force started the front wheel oscillating out of balance. Could have been the wind, an errant slipstream from a passing car, uneven pavement, or just some small steering aberration, but it was terrifying. To my knowledge, it had nothing to do with the bike itself; subsequent descents of the same stretch of road went off without a hitch.
You tell a great story, by the way. Glad you lived to tell it!