server load more than 1000 (one thousand)

How can that happen server load more than 1000 (one thousand)
PHP Code:
top - 15:20:21 up 9 days, 15:33,  2 users,  load average: 545.95, 732.81, 938.91
Tasks: 3326 total,   2 running, 3319 sleeping,   4 stopped,   1 zombie
Cpu(s):   6.2% user,  21.1% system,   0.0% nice,  72.6% idle
Mem:   1029740k total,   981664k used,    48076k free,    48948k buffers
Swap:        0k total,        0k used,        0k free,   445128k cached 
I couldn't capture when it went over 1000.

After 2 restart http, 1 reboot server, and then cold reboot, everything seems ok now. But I dare that it will re-happen some day.

 

 

 

 

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