Caulk can sometimes crack within it's thickness because it's too old, it's past it's shelf life. If that's the case though it usually comes out of the tube a little funky. When caulk cures, it shrinks. If it's put on too thick the shrinkage forces can sometimes be so strong that it pulls off one of the adjacent surfaces. I've never seen it crack down the middle due to this, just pull off and separate from one of the surfaces. It can happen months, even years, later.

Old caulk can cause problems. I've never seen caulk "crack" though. I have seen, when it is put on too thick, too much at once, or when it's put on an unclean surface, or put on a surface that was previously caulked with 100% silicon... In those instances I've seen it pull away from one side of tile. Normally due to shrinkage as it cures. But I've never seen the bead itself crack. For deep gaps you can use foam backer rod to "fill" part of the depth of a deep gap. Then you can caulk over the backer rod. New construction it could be house movement, caused by wood shrinkage. The baseboard is shrinking in the dry, less humid winter months, and "pulling up" away from the floor. If the caulk failed due to shrinkage, then you can sometimes go over it with new caulk and blend the two applications together. But it's hard to diagnose if it's excessive house movement or bad caulk. Since you've caulked before it's probably not operator error. If the caulk was bought recently at a store with good product turnover then the caulk was probably fine. So it could just be the first-year construction settling in during a dry winter.

1. Never sand caulk. It doesn't sand. 2. Let it cure for 30 days. That will allow plenty of dry time. Apply a thin caulk over the cracks and let dry 24 hours. 3. Repaint over it. Michael

if you go to all the trouble to take the siding off to really fix this long term. invest in a quality flashing tape, not a cheap brand. I'd hire someone with vinyl siding install experience & have them remove the siding to expose the sheathing of the wall. then hire someone who understands flashing methods. you can learn & show them how to do it properly, and perhaps even the vinyl removal person can do the flashing. best of luck.