To some degree, it's voodoo. When you deal with machines of a particular OS every day, you get a feel for what the numbers mean.
you can't determine why as (no processes are using high CPU)
Roughly, the load avg is the length of the run queue (how many processes are waiting to be run or are being run, vs. how many processor cores are available) with some IO activity thrown in for good measure. It could be a lot of IO wait or a lot of small processes that are driving up the load avg -- not necessarily just one process trying to hog CPU time.
There are a lot of good suggestions here -- many sysadms are too reliant on load avg (myself somewhat included).
you can't determine why as (no processes are using high CPU)
Roughly, the load avg is the length of the run queue (how many processes are waiting to be run or are being run, vs. how many processor cores are available) with some IO activity thrown in for good measure. It could be a lot of IO wait or a lot of small processes that are driving up the load avg -- not necessarily just one process trying to hog CPU time.
There are a lot of good suggestions here -- many sysadms are too reliant on load avg (myself somewhat included).