Secondary voltage is heavily determined by load. The voltage goes up to satisfy demand. If the circuit is deficient the greater the voltage the more likely a break down.
During cold running the possibility of moisture related shorting to ground reduces the voltage necessary for such.
The combined properties add up to the highest likelyhood of secondary break down being cold. Load is highest and resistance is lowest.
Bosch Master, ASE Master, L1
33 years MB technician