My phone just started screaming.

I was using my computer, didn’t touch my phone. Suddenly my phone started screaming more and more loudly. It was looping over and over again and I couldn’t stop it, I tried lowering the volume and just saw how the volume bar on the side rose.

I rebooted my iPhone and it hasn’t happened again. Not going to lie I got pretty scared, what caused this to happen?

Thanks in advance.