Pretty much just normal shrinkage in winter as things dry out, possibly aggravated by using damp wood or not running A/C in the hot/wet summertime - so if replaced/installed/painted before you moved in in the summer, it would be swelled up, and as you run heat and indoor humidity falls in the winter, the wood shrinks.

Personally I'd suggest waiting through a year or two before fixing it unless it's the top of your spousal unit's issues with life in general or the house in specific - once it's "cycled" a few times it will move less (not none) in the future so a fix will be more likely to stay fixed. But's basically paintable caulk and paint to fix it, nothing major to get excited about.