iPhone is locked for 15 minutes

If you routinely walk with your iPhone in your pocket, I'd recommend you go to Settings > Accessibility > Touch > Turn off Tap to Wake

This will prevent your phone from letting the screen be interacted with without your direct input. This will change how you wake the screen though as you'll need to tap the power button to awaken your iPhone. It's a small price to pay for possibly getting locked out of your iPhone for an even longer time frame or even possibly disabling the phone, which would require you to erase it using a computer.

The phone was in your pocket or purse. It timed out and then it rubbed against something.....which was entered as an incorrect passcode.

Since the passcode was incorrect, you will have to wait for 15 minutes and then enter the correct passcode.

It might help to make sure that the phone's screen faces away from you if you have the phone in your pocket.

You kind of did enter the wrong passcode. By having the phone in your pocket, your movement woke the screen, which then continued to enter a passcode (incorrectly) with more movement. This is why I recommend you turn off Tap to Wake if you are in the habit of having your iPhone in your pocket when you are out walking.

Careful.......if the wrong passcode is entered too many times.....you will be locked out of your own phone.