Some ancient machines trapped on signed overflow, including the ones caused by arithmetic shift left (as opposed to logical shift left), and apparently you can't define your implementation-defined behaviour as "oh, it traps in this case" — although still I don't believe the last part.
But if it is true, then yeah, that'd make (some) sense. I say "some" because surely in this case the implementations could simply emit the "logical shift left" instruction for the shift operation both for ints and unsigned ints? I don't think there ever was a machine that actually tracked the signedness/unsignedness of registers.