Many of the elves are out sick and the part-time elves aren't working as fast as the regulars, so they're falling behind schedule for production and Christmas Eve is rapidly approaching. 

When Santa went to pre-load a bag of toys into the sleigh, his foot went right through the bottom of the sleigh, the floor rotted through, needing replacement.  More work to do before Christmas Eve.

And, he noticed more than half the reindeer absent from their stalls, out somewhere gallivanting around who knows where, and that most of the female reindeer where in the "family way", certainly unable to fly.  He'd have to head out across the North Pole to round up all the reindeer to put together his team for Christmas Eve.  Yet again, more work to do on a tight schedule.

Also, Mrs. Claus was complaining about his working all these extra long overtime hours every single night.  Then, she informed him her mother is coming to visit for the holidays.  Not a relaxing family visit. 

So, Santa figured he'd have himself a drink of the hard cider, and goes to the cupboard where it's all kept.  Only, he finds that every jug is empty (now he knows why his elves are out sick).  And in the process of checking all through the jugs, he dropped one, it smashing into a thousand tiny bits and pieces on the floor.

So, he goes to get the broom to sweep up the mess, only to find out that mice ate up all the straw on the broom leaving it as just a nub on the handle.  Nothing seems to be going right.  What more could go wrong?  

Just then he hears a knock at the door, "Tap, tap, tap, tap." 

He goes to the door and opens it wide to discover this beautiful little angel standing there holding a pretty big nice Christmas tree.  And the angel exuberantly, cheerfully, and excitedly says, "Merry Christmas Santa.  What a fabulous day!  Isn't this a just beautiful, wonderful tree for Christmas?  ...where would you like me to put it?"

And so therein began the tradition of having a little angel at the top of our Christmas Trees.

Now, if you doubt the validity of this story, just remember:

we NEVER see Santa Claus as part of the Nativity Scene, and 
we NEVER see angels help Santa and his reindeer on Christmas Eve.