I've noticed a recurring pattern when I do sustained hard runs in which my heart rate increases in an expected, gradual way during a hard bout (e.g., running uphill for an hour or doing a 30-min set at my tempo pace), but then starts to increase much faster after the hard bout is over (e.g., running back downhill or returning to a moderate pace after the tempo set). Then my heart rate stays much higher for the rest of the workout, even though the hard bout is long gone.
For example, on a 2-hour trail run that is initially uphill, I might average 145-150 bpm on the way up at a fairly hard effort, but then average 170 bpm on the way back down. Or, here's an interesting heart rate graph I saw during a tempo run:
The red line is my heart rate. (Yes, the axis scaling is ridiculous, and I don't know how to change it.) You can see how, from about the 2-mile to the 6-mile mark, I had entered a main set of tempo running, and my heart rate steadily climbed from 143 to 157. After that, I went down to a moderately easy run, yet my heart rate quickly climbed to 193! (The fluctuations at the end are because I was alternating 5 min of easy running with 5 min of tempo-pace running.) And interestingly, those last very short sets of tempo running also felt really hard -- definitely harder than they would have felt if I had just extended the main set by 10 min.
Interestingly, I do not see this heart rate pattern when I do much shorter work bouts (e.g., V02 max intervals of ~3 min in length). Then, my heart rate goes up during the work sets (to, e.g., 155-160ish -- not to 190!) and then down during the recoveries. Also, if I do the same trail run but take the uphill at an easier pace (e.g., heart rate 135), then my heart rate goes down, as expected, when I start heading downhill again.
What would explain this pattern of delayed heart rate increase after a hard bout?