The first thing that Paul says can cause us to become unplugged is grumbling. This is something that the Children of Israel did a lot of. So what is so bad about grumbling that we get disconnected from the Power Source? Let's look at a couple definitions of the word. Webster says - to complain quietly about something : to talk in an unhappy way
While Dictionary.com says -
1. to murmur or mutter in discontent; complainsullenly.
2. to utter low, indistinct sounds; growl.
So when we are grumbling about something we are complaining under our breath that we are not happy with what is going on. God would rather for us to come to Him and tell Him directly than to grumble under our breath, hoping for Him not to hear.
The second thing Paul says disconnects us from the Power Source is disputes. Have you ever tried to argue with God as to why you are not the one or why you shouldn't do something. I have. I fought with God for along time as to why I'm not the one to be writing for Him, obvious I lost. I'm not alone there are others in the Bible the tried arguing with God and lost, like; Moses, Jonah, and Peter.
Not only when we argue with God are we telling Him that we don't like His plan, we are also telling Him that we know what is best. We have unplugged ourselves from the Power Source and become dependent on our own power, when we argue with God.