@cheesefest - why do the chillers stop working, do you know? I wouldn’t have thought the outside temperature would matter as much since most stores are air conditioned?
From what one of my friends who worked at Tesco for around 20 years said, probably poor maintenance/putting off repairs/upgrades because they're "coping" (barely) in normal weather, then when the heat hits the extra work kills them.
IIRC supermarket coolers/freezers tend to work from "common" large scale heat exchangers similar to building AC as it's far more economical than having each unit running from it's own, and can mean you say have a spare in the circuit so that if you suffer a failure it can fall over to that (think raid 5 vs raid 0 or 1) without losing the contents of the coolers.. The problem is it's costly to maintain them, especially preventative work so if the store manager is trying to be "efficient" and "save overheads" he/she may decide to risk putting it off, especially if doing the work requires shutting down a section of the freezers.
Even if the store is airconditioned, the heat exchangers for the chillers are usually a large unit outside of the cooled store (why would you pump the heat from them into the store), so still have to cope with the outside temperature which if that's ~5-10c more than usual means it's going to have to work harder to dump the heat, and if your unit is already near the maximum output *under normal conditions it's going to start to fail.
My friend was telling me a while back about how his store ended up with several freezer trailers parked up blocking a large part of the warehouse/loading parking area for months because his management had put off doing work on the aging chillers for the warehouse, and when they failed they were stuck waiting for the parts to be available (I got the impression the failure required a lot more work to put right, than what would have been needed to prevent it, as is usually the way).
I suspect the use of the freezer trailers probably cost the store a small fortune.
*Due either to lack or maintainance, or you've increased the number of "cooled" units and eaten up the exccess capacity that might have been built into the system to allow for really hot days.