Why are these cracks around all of our sash windows?
Silicone should NEVER be used on painted trim. 

The separation that I see here can either be seasonal movement, as in the trim was installed in the summer and now it is winter where the moisture content of the wood is much less, therefore making it shrink. 

It also could have been installed with too high of a moisture content to begin with and the loss of moisture is now evident. It may even be as simple as not enough nails used at the proper angle to hold the trim fast in the corner.
 Since it is a touch up item, I would add a few piloted 2 1/2"-3" nails angled into the corner to tie it together, set them below the surface and putty and sand flush. Before you commit the nails, make sure the trim is solid in its backing. You will notice it if it starts moving while starting the nail in, the trim will want to compress in. If it does, it will make the gap MUCH bigger. If so then you may want to use a trim head screw.

After it is secured, caulk with a good grade of painters caulk, it can be siliconized, but not 100% silicone. Prime and finish paint.