Did the chest strap slip out of place on the descent, then you adjusted it? Once happened to me running fast down Arthur's Seat, the strap slipped down off my chest completely and instead of reading zero, it gave erratic readings well above my Max HR, similar to what your plot shows.
Can also be a sign that the battery is starting to go and isn't consistently transmitting a strong enough signal.
I reckon you're just unfit
actually i was just thinking about you at that point Nixon
Looks like you went for seconds on the 4th one too...