Why does iPhone becomes "unavailable" for no cause?

No, it isn’t a software and hardware problem. Go to Settings/Display & Brightness. There’s a switch called “raise to wake”. If it is on when you raise the phone to a vertical position the screen wakes and waits for your passcode or Face ID. If you carry the phone vertically in your pocket it will “think” you have raised it to unlock it and wait for a passcode to be entered.

Did you check the setting of the Raise to Wake switch? If it is on, after you lock the screen when the phone is in a vertical position the lock is overridden.

It wasn’t “for no reason” - it was for a very specific reason. While it was in your pocket you "butt-pressed" the unlock code buttons too many times (about a dozen) and the phone became disabled.

Hi Arandel,

Since having the latest software can help with overall performance, make sure your iPhone is running the latest version of iOS 15 (15.5). It's also a good idea to check the firmware for your earbuds and update them if there's an update available. If you're able to pair your earbuds with another iPhone, check to see if you notice the same behavior. This step will help with determining if the issue only occurs with your iPhone.

If updating your devices doesn't help, and the issue only occurs with your iPhone, reach out to Apple Support so they can look into this further. They can be contacted here: Get Support, or by calling the most applicable phone number from this list: Contact Apple for support and service.

If you notice this behavior when your earbuds are paired with multiple devices, contact the manufacturer of your earbuds for additional guidance.

Best regards.

Pressing the side button doesn't make the screen inactive, it puts it to sleep. Per this article, Use the side, Home, and other buttons on your iPhone - Apple Support, "press the side button to wake your iPhone or put it to sleep".

Wake and unlock iPhone - Apple Support