I think little ones act without thinking, and when we question them as to "Why did they do this" or "What were you thinking" is the moment they realize they did not think this action through and they are embarrassed or worried about us being dissappointed in them.
As big people we need to be careful with our tones in our voices so the little ones don't feel threatened and made to feel stupid. Think about when a friend does something and you ask "What were you thinking" and the same statement and tone you might use when speaking to a "little one".
We need to remember that even as adults we screw up...........most decisions are based on the knowledge we have at the time we make the decision...........little ones have alot less life experience to draw from.
So in a nutshell, children sometimes lie because we have frightened them into do so. (Not all the time, sometimes they are just mimicking us)