Hi Andrew!
[Please read through to the end, as I realize I misunderstood your question in the first part of my response!]
The short answer to the query in your title is, "No, they don't all do that". In fact, I'd say by the lack of user reports, Thorns probably shimmy (castor) less than some other makes.
I had a similar problem develop on my Sherpa and tied myself in knots looking in vain for causal factors before sending the bike back to Thorn and Andy Blance, who tested it by loading it heavily and riding it at speed downhill with no problems. It was the only one of my many bikes ever to develop this trait, and my Nomad certainly does not, nor did the Raven Tour very kindly loaned me by Forum member AndyBG for my 2014 double-crossing of all Europe. It is worth noting the RT had essentially the same frame as my Sherpa Mk2, yet had no shimmy at all. I must also note Thorn did everything right in resolving my warranty issue and I could not have been more pleased with their response and how seriously they viewed the whole situation.
I've come to the conclusion some shimmy factors are and will remain simply unknown, as they did in my case.
However, having said that, there are some well known factors that contribute to shimmy for many of the people whose bikes suffer from it. Among the factors to check...
• Load weight and distribution
• Type of rack... a rack that is not rigid can actually induce sway that leads to castgor at the front of the bike ("tail wagging the dog" effect)
• Rider positioning
• Headset adjustment/wear or type -- low-trail randonneur bikes tend to be more subject to shimmy, and the greater friction of rollerbearing headset has solved many cases. I have these fitted to all my bikes except the Nomad (and late Sherpa).
• Frame geometry -- not just in absolute terms, but for intended use.
• Tube length -- bigger frames use longer tubing and are less rigid.
• Frame size (see tube length above)
• Tube diameter
• Tube wall thickness
If a person tries to tour with a too-heavy load on a too-light bike, the limits of good handling will be crossed at a lower threshold and one is more likely to develop problems.
In general, Thorn frame geometry tends toward high trail, which also favors good handling with rear loads. The RST is built with less robust tubing and tighter geometry than its Raven Tour stablemate of the day, and it may be it simply does not tolerate a heavier front load so well before the handling deteriorates -- that's due to the "Sport" focus in the RST name.
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Having just written the above, I'll let it stand, but going back and re-reading your post once again, I think the problem you're describing comes not while riding but while at rest, and is more commonly referred to as "wheel flop". This characteristic tends to manifest more on bicycles with higher-trail geometry.
If this is what you mean, then yes...my late Sherpa, my current Nomad, and AndyBG's Raven Tour all manifested it to some degree.
I have found it can be easily controlled while at rest atop the bike simply by locking up the front brake so the bike cannot roll once the handlebars turn themselves to one side. I use and prefer Bike Brake parking brake bands because they are durable and resist UV /ozone/stretchout breakdown well and maintain a low profile against the handgrips/bar tape. You can see them here:
http://www.bikebrake.com/ An alternative is to simply cut and slip some sections of innertube over your grips to accomplish the same purpose. They won't last as long, but will do the job for a good while and would tell you if they help the problem of the bicycle taking wheel-flop forays while at rest.
All the best,
Dan.
