It depends a lot on what the machine was doing at the time.
In these instances, Activity Monitor can give you an idea of what is using so much RAM.
Web browsers with lots of tabs open can use a huge amount, and sometimes a poorly programmed website can cause the RAM use to spike.
Of course it could be something else, I am just pointing out a possible cause.
Activity Monitor, click on Memory; make sure that it is sorted by Memory in descending order.
Also, very important, check the "Memory Pressure" at the bottom. Usually it is green, indicating that things are ok.
