<==== Ex-“Tanker Yanker” here…

I can see your theory, Munchman, and it makes sense…but…there is one thing which causes more tanks to end up on their sides than anything else, and which is probably least widely understood, and that’s SLOW SPEED ROLLOVER!!!

It’s not speed which causes the problem, it’s smoothness (or lack of it)…in the case of a motorway crash like this, an emegency overtaking manouever - for example if some prat slams his/her anchors on in front of you and you have to leap into the outer lane whilst braking to avoid hitting them, and then back in quick to avoid the guy you were forced to pull out on hitting you (we’ve all been there, can be a lifesaving move all round at times) - could well be the trigger… 
I’m a lazy [zb] at heart, so I’ve dug up this post which I put up last time we had this discussion which explains Slow Speed Rollover reasonably well IMHO (although if someone can do better, feel free). 
Personally, I think a full understanding of it should probably be included as part of your training when you get your wagon licence (since it can happen to many vehicles with many types of freight)…and certainly be compulsary before getting in a tanker of any description, particularly one carrying HazChem. A tanker in the head will kill even the most solid of car drivers!!! 
SLOW SPEED ROLLOVERS
I was taught about the dangers of slow-speed rollover when I first did tank work. Tankers, containers full of loose freight, and hanging meat wagons are, as ever, most susceptible… 
It’s difficult to explain but without using diagrams, but I’ll give it a shot… 
It’s most likely to happen on either roundabouts or a series of s-bends…I’ll take roundabouts for this example, and use a tanker as our hypothetical vehicle, for ease of explanation.
Think about where the liquid in a tanker goes as it takes a roundabout, a step at a time…
- Tank driving straight. Load is fairly stable, err-ing towards the back of the tank due to a combination of inertia (of the load) and forward movement (or the lorry).
- Tank brakes as it approaches roundabout. Inertia means that the load continues to move forward, ending up at the front of the tank.
- Tank accelerates onto the roundabout whilst steering left The load heads for the back of the tank again, this time into the right hand corner.
- Tank steers right around roundabout Load stays at the back of the tank, but goes into the left-hand corner.
- Tank steers left to exit roundabout The load goes back to the rear right-hand corner again.
- Tank straightens out after leaving roundabout The load settles centrally at the back of the tank, and all is well.
That, of course, is the ideal. Notice that apart from when he first approached the roundabout, our driver didn’t brake. 
If he had dabbed his brakes a tiny tad too hard…well, let’s take the point where he has finished steering left (onto the island) and has started to steer right again…
If he braked just at the wrong moment, instead of just going from one side to the other of the back of the tank, the load will go diagonally across the tank…so it goes right to left (because of the change of direction) and back to front (because of the braking)…finishing up in the front left hand corner of the tank…
It is this diagonal serge into the “shoulder” of the vehicle which can make it flop, even at very low speeds…I kid you not, a fitter from the firm I was on when this was explained to me took a loaded tank out of the yard and put it on it’s side at less than 20mph as a result of this one…no-one’s fault, just lack of knowledge on his part and unfortunate timing with the brakes! 
How to avoid it? Take all roundabouts slowly, and above all, smoothly if you are pulling a trailer which contains any type of “loose” freight - or a rigid of the same ilk, for that matter…and try and spot the cars cutting 'round which might make you brake early enough to do it gently. 
Hope that makes some sort of sense… 