Enforced updates?
Just spoke with our signing guru, and most signing tools enable time-stamping on a signature by default; this would prevent the signature expiring when the certificate expires. The signature is checked against that point in time, whereas a non-time-stamped signature checks 'now' against the certificate's valid period. (Apologies if something has been lost in transmission.)
So the expiration is from a, by choice, non-time-stamped signature.
Now, why would a company choose for their signed software to expire?
Because even Paris would use the defaults.