iPhone is disabled - try again in 1 minute

This morning, while listening to a podcast on my iPhone, I walked (30 minutes) to the store, shopped for about 15 minutes, and then headed home. Halfway back, my podcast suddenly stopped. I pulled my phone out of my back pocket and saw "iPhone is disabled - try again in 1 minute" emblazoned on a black screen. "Odd...," I thought. Having walked with my iPhone in my back pocket for years now - upside down, so as not to activate the screen - I've never experienced this before. I brushed it off as a fluke, turned my podcast back on after the minute had passed, placed it back in my pocket (upside down), only to have it lock again within a minute or two. (This time, requiring a 5-minute wait.)

I'm less concerned about how to deal with a disabled iPhone than I am curious about why this might be happening. Any ideas as to the cause? Is my back pocket too roomy, causing the screen to activate, even if upside-down? Is the latest Apple update making the screen more sensitive now? Could it be the new (non-Apple) ear buds I am using? Any ideas of how to avoid this kind of thing in the future? Anyone else experience this in similar conditions?