Some modules need considerably more power than stated in their specifications when they are first starting up. This start-up burst is referred to as Inrush Current (as well as input surge current or switch-on surge). Tube modules often warn that they need twice as much power when first powered up. Modules and bus boards that have more than a one or two electrolytic capacitors also need additional current, as well other modules that flash all of their LEDs when they start up, etc.
More sophisticated power supplies have protection against too high of a current demand, and will shut down the voltage rail where it senses too high of a demand. As a result, you may need a much larger power supply than you think just by looking at the ModularGrid ratings.