I don't know why you find it surprising. It is certainly not the case that a rear end is automatically negligence in any state.
True, but I think CT got it bass ackwards. Should be a presumption of negligence that the defendant should have to rebut.
The interesting part is that the company prolonging the litigation was the plaintiff's own insurer. This is an odd situation where they argue the party they weren't insuring was not at fault.
That's because underinsured motorists coverage (and uninsured motorists coverage) is an adversarial coverage between an insured and his own insurance company. The plaintiff's own insurance company stands in for the other driver's insurance shortfall and only pays if the other driver is negligent.
The UIM insuring agreement:
"We will pay compensatory damages which an insured is legally entitled to recover from the or operator of an underinsured motor vehicle because of bodily injury sustained by an insured and caused by an accident."
If the other driver is not negligent then the insured is not legally entitled to recover from the other driver. The insurance company has good reason to defend the other driver.