It looks like you are on the right track. As for figuring out why this happened I can offer you this advise:
You focused on an object as you entered sleep again. If you focus on a clear image right after awakening or during this half asleep state and try to calmly reenter the dream you will find yourself in the image you were thinking about. The fact that you are in the setting you imagined works as a RC, instantly making you lucid.

The trick here is to focus on one image, otherwise like you said you will enter some random dream. Thinking about a specific setting will make you enter a dream about that setting.

I use the image of two particular classrooms that are also my dreamsigns - I simply reenter them fully lucid.