In Windows 8 many things have changed, one of these changes was the introduction of a new model of modern apps. Windows 8 also included a new process called Runtime Broker with the main goal of managing permissions inside these new modern “Metro” apps. Normally this new process should only use several megabytes, but in some cases it could spike the memory usage to gigabytes.
The problem is related to how the system handles the live tiles in the Start screen. This happens when a tile update request set aside a portion of RAM and never returns to the main system memory. When this continues to happen during a certain period of time, the memory allocated keeps growing to the point that your PC runs out of memory.
Microsoft already knows about this issue, but has yet to release a fix. Now, until a solution arrives for the Runtime Broker problem and if you experience sudden high memory usage in Windows 8, you can either try to end the process from the Task Manager (Ctrl + Shift + Esc keyboard shortcut) or uninstall any Windows 8 app that you think is causing problem — Restarting the PC will also help.