Answer from Tom Baker:
Peroxides can undergo self-accelerating thermal decomposition and are kept a low temperature to avoid this. If not controlled the decomposition can be violent; this quantity of organic peroxide has blown the doors off a refrigeration unit before. The bulging containers could be a sign that decomposition has begun, the best way to tell is use a thermal camera to see if the containers are hotter than their surroundings, your local fire service will have one. Otherwise, they’ve bulged as they warmed to ambient temperature and are safe to vent. But take care, the containers will be pressurised and may spray out their contents if opened too quickly.
Tom Baker is a senior responder at the National Chemical Emergency Centre