Hmm, as ededwards said, hard to pin-point.
Could you try and repeat it to isolate the problem, or to give you confidence there isn't an issue.
Not the thing to do in a race, but mabey worth trying while training.
I'll probably do that very thing this weekend if I've time as the stretch of road involved is a short ride from my house. Being really scientific about it, I probably ought to do two downhill runs, with and without tri-bars.
I once had a really bad speed wobble episode whilst in the Yorkshire Wolds on a previous bike (due to its back wheel not being dished correctly), which I got out of by using the gravel trap at the bottom of the hill intended for motor vehicles. Came out of that one unscathed but did fly over the bars and land in the gravel on coming to a dead stop!
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