Voice cracks, voice breaks, you name it.
During puberty, your vocal cords are switching to a new vibration. When in transition, the rapid switch between vibrational levels causes a squeal similar to someone practicing a new note/chord switch and hitting it wrong.
Outside of puberty, stress (when angry or emotional) can remove focus from normal speaking technique and cause a similar squeal (also can increase cracks/squeals during puberty like when talking to a crush or during argument).
To add onto the other answers, coordination plays a role as well as changes in physiology. The range of sounds required in human speech is pretty large and it takes a serious amount of coordination to make your anatomy do the right things at the right times to make the correct sounds.
The sound begins in your vocal folds, travels up your throat, and goes out your mouth and nose. So your vocal tract is basically a tube with a bend near the open end. There’s a physical phenomenon in acoustics which is demonstrated in [this video (demo at 1:20)](https://youtu.be/F64xcPKKES8) that also applies to your vocal tract; basically, a property of tubes is that certain pitches can’t resonate through them as well as other pitches. So as these people try to vary the pitch of their voice with the tube against their mouth, you can hear what is essentially a simplified voice crack (or a yodel). The pitch at which the yodel happens depends on the length and diameter of the tube.
Obviously your head and neck are way more complicated than a solid tube, so the pitch at which your voice cracks happen aren’t fixed. The position of your anatomy affects where the cracks happen. As you go through puberty, you gain practice avoiding these positions (because it’s really embarrassing, and people tend to hate feeling embarassed) and thus avoiding the cracks.
If you mean why, then it’s a one-word answer: tension.
Your vocal cords aren’t “cords” at all, they’re actually folds. It’s a lot like the double reed system you see on instruments like oboes. In order to produce speech, muscles tension and relax the vocal folds as air is blown through them from the lungs. The air causes the folds to vibrate, giving you sound.
This system is easily upset. If the vocal folds are inflamed (say from yelling a lot or because you’re sick), if there’s excess mucus in the larynx (where the vocal folds are), or if you’re nervous, the ability to properly tension the vocal folds and getting them to vibrate correctly can be impaired. Too much tension or vocal folds that are too inflamed won’t vibrate correctly and can stop vibrating altogether as air passes through. When the vocal folds have an interruption of vibration, you get a break. If the muscles spasm and suddenly put the vocal folds under too much tension, you get a crack.