Locking display buttons

Had my iPhone 6plus since Oct 2014. It has worked fine until today.

The phone was in my pocket, I had BT earphones (Nokia BT 505) on. I had locked the display buttons by pressing the wake up button before putting the iPhone in my pocket.

Had the Nike + running app on and was listening to music. Then I received a call, announced by ringing through the earphones, and activated the call by pressing a button in the earphones. The call went on normally for a 10 -15 minutes. Then I started hearing irregular touch tone beeps, which were also heard by he caller, but the beeps did not originate from his iPhone 4. Then, there were two vibration signals from the phone in my pocket and the call dropped. I took the phone out and saw a text on the display (in Finnish) "The iPhone is disabled, try again in 11 minutes".

And sure enough, after 11 minutes I was able to make call.

What is the explanation ?

Did the display buttons unlock and become active by themselves in my pocket giving rise to the touch tone beeps that I heard, and the erratic display button activity leading somehow to the phone being disabled ?