It is because your computer cannot cope from the load. It either seems that the RAM is being bottlenecked or the CPU.
Please provide specs of your RAM and CPU
Also try closing unneccesary programs in Task Manager under processors. If you can't distinguish between neccessary and unnecessary programs I suggest you download Game Booster Pro v3 Beta and install that program, click boost and it will close unneccesary programs freeing up RAM.
Also download CCleaner and run a scan, then click ok, ok, and delete the useless junk sitting on your computer. It deletes useless stuff like temp files and internet history