Why does a clivia bloom when its flower stalk is very short?

This appears to be the "stuck arrow" phenomenon. There are generally two causes: one is insufficient fertilizer and water, as the potting soil lacks enough moisture and nutrients, so the flower stalk has no strength to push out. The other is that the ambient temperature is not high enough. The optimal temperature for the flower stalk to emerge is 20-30 degrees. The solution is to fertilize thoroughly before flowering, especially with phosphorus and potassium fertilizer. If the temperature is low, you can add warmth at the base of the pot.