AC adapters are generally engineered to function despite fluctuations in the mains supply that they're connected to. But it's possible that in this case the voltage periodically falls too far out of spec, whether due to problems in the building's wiring or issues with the power company, both of which are more likely to be manifest when it's hot and demands for electricity are higher. It's also possible that one or more of the devices in the chain - extension cord or power strip, adapter, watch charger - are marginal in some way and therefore more susceptible to changes in the power supply.