Why do I get into power struggles with my child?

As I've briefly mentioned before, when a parent gets into a power struggle with their child, it is due to the parent's wrong goal of trying to enforce a behavioral change upon their child.  And the thing about power struggles is that they start out not seeming like anything more than a case of my child just isn't clear on what I want him to do or he just isn't listening closely.  You know this routine, I'm sure.

Little 6 year-old Sally is playing in the living room.  Mom (or Dad) says, "Time for bed.  Start cleaning up your toys and get on your pajamas."  The little girl says "OK", but keeps on playing.  A few minutes later, in a somewhat more serious tone, the parent calls out, "Sally, I told you it's time for bed.  Now clean up and get going!"


And the dance begins in earnest.  It takes two to tango...

"Mommy, can't I play for just a little more?" 
"No!  You heard me.  I said clean up your toys now."
Sally, putting on her mopey face, starts making a rather half-hearted effort at picking up her toys... very, very slowly... leaving some here and some there.
Mom's patience is now running very thin.  Time for Mother to bring some real motivation to bear on the situation.  In a raised and this-time-I-really-mean-it voice...
"Sally, if you want to play with your toys tomorrow, then listen to me now.  Get moving and put those toys away this instant!"
A little more movement ever so grudgingly, as the half-hearted effort of Sally ramps up to about half-hearted-plus-one.
Mom now enters the living room again and, this time emptied of patience, personally points out each toy and telling the child, "Pick this up.. another one over here... that's not where it goes... you know where it goes... put it on the shelf...", etc...

The whole episode continues like this into the bathroom and bedroom until Mom, now angry at Sally for not obeying, has issued  issued threats of a spanking... or actually has administered one along with oft-given and suitable mini-lecture about listening to Mommy.  This is all accompanied by tears, I'm sorry's, and Mom feeling bad about the whole episode.

What just happened and why?