As has been outlined.
There are multiple causes as to why your heart rate will continue to increase with a constant effort.
Ultimately the notion of "decoupling" is the human need to simplify complex topics into understandable chunks. It never is 100% conceptually correct, but as with horseshoes and hand-grenades can often be "close enough".
It could be that dehydration is lowering your blood volume which lowers your heart's stroke volume (how much is pumped per beat) which causes an increase in Heart Rate. (Less pumped with each beat = more pumps to achieve the same output).
Typically, however, decoupling in someone with reduced aerobic fitness is due to Muscle Fiber utilization.
Slow Twitch fibers are rather efficient. More so than their Fast-twitch counterparts.
Therefore, you begin exercise at a low to moderate intensity and slow twitch fibers are the primary contributors to moving you down the road. However, in a low aerobic fitness individual, these fibers will soon begin to fatigue. To compensate, additional fibers begin helping out... fast twitch fibers. Because they're less efficient, increased cardiac output is required to maintain the same workload. This causes "decoupling". In more-fit individuals this is less likely to happen as the slow-twitch fibers are better adapted and presumably more "fatigue" resistant. (Fatigue is a completely different topic)
Now, in certain instances, glycogen depletion due to 2 - a -day workouts etc may cause individuals may be more prone to decoupling.
Therefore, you cannot necessarily hang our hat on any one reason. You need to look at the trend of the situation.
If you always decouple, it's probably an aerobic fitness situation
If you rarely decouple significantly, but do so on hot days where you didn't drink enough, it's most likely a dehydration / blood volume issue
If you decouple after a fast or period of low-carb eating, it could be glycogen depletion.
etc.
etc.
I talk a lot - Give it a listen:
http://www.fasttalklabs.com/category/fast-talk I also give Training Advice via
http://www.ForeverEndurance.com The above poster has eschewed traditional employment and is currently undertaking the ill-conceived task of launching his own hardgoods company. Statements are not made on behalf of nor reflective of anything in any manner... unless they're good, then they count.
http://www.AGNCYINNOVATION.com