My vote goes to wheels not running true and vertical. Toe-in/out not so much effect other than to make it harder to pedal and sapping forward motion and scrub tyres.
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See at 19minutes the "twitch", but watch the whole thing because it is really well delivered and fascinating.
This is my naïve level of understanding of the process.
The faster a bike wheel turns the more gyroscopic force tries to straighten it (which allows you to let go of a rolling 2 wheeler and it carries on going, or assists you in riding "no-hands" on a 2 wheeler DF bike).
In a tadpole with 2 linked front-wheels the wheels are in harmony/opposition and these forces are trying to maintain this status-quo.
Knock them out of kilter by hitting a rock/bump with one wheel and as the yare linked by a tie-rod both of them "twitch" and the gyro force tries to straighten the wheels and this destabilises the entire system and it has to try to re-stabilise. The level of force in the gyro is directly linked to the speed of rotation. If one wheel gets out of synch with its partner its attempts to re-establish its own smooth running destabilises the partner wheel which then tries its own re-stabilising which then...(rinse-repeat) and off we go..."Death-Wobble".
I would check the vertical alignment as well as toe-in/out.