The most likely cause is focus. If you focused on the foreground, the stars were probably out of focus. At 24mm and f/4, depth of field may not be enough to keep both a nearby foreground subject and stars at infinity acceptably sharp.

For star trails, focus on the stars (or as close to infinity as possible), not the foreground. This can be tricky because stars are dim, so a practical method is:

1. Manually focus as accurately as you can toward infinity.
2. Take a short test exposure.
3. Zoom in on the image on the rear LCD at maximum magnification.
4. Adjust focus and repeat until the stars look sharp.

Once the stars are sharply focused, shoot your full sequence for stacking. If you want both foreground and stars sharp, you may need to stop down more or use a different focusing approach, but based on your settings and description, foreground focus is the main reason the trails look blurry.