1. You worked out very hard and your muscles are tired
2. You need to eat and replenish
3. You didn't eat enough before hand and you need to eat ASAP...
I sometimes get a little shaky during lifting, usually at the very end when I decide to do one more set past what is wise, or at the beginning when I haven't eaten enough.
I noticed I was a bit shaky when I first started lifting with the free weights, especially bench press. It took me awhile to get the strength to not only push the bar up, but keep it steady. That's where my lifting partner came in very handy. Helped with the control while I was lifting.