Every once in a while, I'll have a time when no matter what I do my voice cracks even though I'll be singing like I usually do. What causes this? Am I singing too much? Does my excess mucus problem add onto this? How can I prevent this from happening, or lower the amount of times it happens?
The voice box essentially has two major stable configurations, chest voice and falsetto. One controls pitch by tensing the muscles ending in the vocal folds themselves: that's chest voice. One controls pitch by drawing the vocal folds taut from their ends using some sort of hinge mechanism. As chest voice gets higher, the hinge mechanism gets closer to accidental engaging, like a trap door getting blown open by wind.
The solution is control. This is particularly done by practising your falsetto and extending it downwards, by practising singing across the break (so that your vocal control is able to immediately adapt) and ultimately reaching the skill and strength where this trap door opens in a controlled manner by engaging the falsetto mechanism not as a response to the configuration change of the larynx but in preparation of it, the way you can stealthily close a door with a sticky latch by pushing with one hand and pulling with the other.
Doing this with not more than the required amount of counteracting forces is the key to sustainable singing.
Your voice can crack because of one of the following reasons:
1) Excessive practice (strains the vocal cords)
2) Singing from the throat and not from the stomach/abdomen.Damages your vocal cords in the long run.
3) Narrowing down, if you use a fake voice that causes pain in the throat . Can get as worse as producing strains of blood in the mucus.
4) Giving too much strain to the voice (Can be due to shouting/talking)
Some tips to maintain a good throat (worked for me) :
1) Hot water with honey in the morning
2) "Tulsi(holy basil)+ ginger" tea
3) Cumin(jeera) seeds in hot water
4) Turmeric powder in milk (if you're lactose tolerant)
Here's the science of voice cracking.
You may already know this, but when you hear a tuning note A of 440Hz, your brain is telling you that you only hear that one pitch — 440Hz, when in the reality, if you were to decompose the pitch, you would find that it is actually the following infinite arithmetic series of pitches, overlaid on top of one another, called the overtone series:
440Hz, 880Hz, 1320Hz, 1760Hz, etc.
When you sing (or play on a wind instrument) a note without proper air support, rather than producing the note you wanted to hear, your voice inadvertently overemphasizes one of the higher overtones in the series.
That's why boys' voices crack during puberty; they're not used to controlling their larger vocal cords, and this improper air-flow causes voice cracks.