Nothing to panic about... you're not doing anything wrong! Leaves are grown, used to feed the plant, and then die, all perfectly natural parts of plant life.

Without going into scientific explanation, I can tell you that it's natural for all plants to lose leaves at intervals. That leaf most likely was old and had completed its task of helping to feed the bulb. It would have gotten weaker and older, then turned yellow and died.

Your bulb will grow more leaves as time goes by... after flowering, it's essential that you provide your bulb with plenty of bright light, and it's time to fertilize the bulb, as well. It will need to recharge for next year's bloom cycle.

Keep an eye on your plant... if it suddenly drops all leaves, then there may be a problem! But if you are caring for your bulb as it requires, losing one leaf is not unnatural.

I had one fairly recently bend over and break a little in the center of the leaf. It was rather firm, I'm guessing it just wasn't quite strong enough to stay up under its own weight.

In this case I looped some string around the upper part to make it take on the U shape it didn't have before. After about a week of growing like that it didn't need assistance any more and stood up just fine since. I've noticed those with nice structural shaping don't have that problem, so they stay up even when they are overly long. Especially those with, for lack of better description, recurve bow shaping seem to stay up better. Sort of a m shape leaf curving.

Yes... that's another thing... without the proper amount of light, the leaves tend to grow longer and weaker than they should... I've had to stake and tie several plants because my windows and cheap lights don't provide enough light for the plants to grow strong... as they would if they were outdoors in natural conditions.

The sun and wind outdoors signal plants to develop a tad differently in the leaf department, and the leaves are then able to withstand nature. Even then, there are leaves that flop under their own weight.