Is this heart rate signature normal? (Or is it a hardware / software / biological bug?)

During the current running season I bought myself a Garmin Forerunner 235 gps watch, which have given me more insights into my running. Before I didn't get statistics of my cadence or heart rate (and was a bit worried that my phone would take damage if it started to rain when out for a run).

Now that I have gathered heart rate statistics for a while, I see a recurring pattern in almost all of my heart rate diagrams. In the beginning of the run the heart rate is rather stable at a value, but suddenly there is a threshold and the heart rate jumps to a stable significantly higher pace.

I think it makes perfectly sense that the heart rate increases during running, but I find that steep rise a bit odd and therefore wonder what could cause it. I suspect that my wrist becoming sweaty after a while can play a role in this, but still, I think that wouldn't show up as that steep in the diagram.

This season I mostly have been running 5 km runs and haven't been doing any warm-up before starting running. 

Here are other diagrams.

This diagram looks more what I would suspect most of the diagrams would look like, but its kind is rare for me.

A seven km run with a jump from 120 to 170+ at around 3 km.

 A five km run with a jump from 140 to 170 at around 3,5 km.

A five km run with a jump from 120- to 160+ at around 1 km.

So, it this anything that looks familiar to you? In that case, any idea what causes the steep rise?