Road Cycling - While in the drops

Here is the observation that I have made:
When I go into the drops my heart rate climbs. Either from my rib cage being compressed, using more leg muscle thus requiring more blood, or I'm unknowingly hammering the crank harder.
If it's the rib cage thing then the bike set up is forcing me flatter than I should. However, The angle of my upper arm to torso is right at 90 deg and my forearm parallel to ground is 90 deg at elbow.
I've tried being more aware of not hammering on the cranks, yet the HRM stills shows a significant increase in heart rate over riding hoods.
I'm leaning more towards the using more of the leg muscle and glut theory.

Anyone else experience this?
Is there a certain conditioning curve the body must go through for riding the drops for long stretches?

I don't know the answer
though I'll be keeping a closer eye on my next rides to see if I experience the same.

I speculate that one reason might be the limited space for lung capacity;
one is forced to take shallow breaths, usually leading to quicker breathing rhythm, leading to higher heartrate.
that of course coupled with the other reasons you stated and any other exhilarating reasons you'd find yourself in the drops.

Could be caused by your torso angle closing, which restricts breathing. Try rotating your pelvis more, instead of bending your back. Think flat back. Another reason is your belly hitting your thighs, which can be cured with some work.
What happens when you are on the hoods, and you just bend your elbows to lower your position?

I have this experience while riding in the drops. I think the reason is that the lowered position restricts your breathing so it reduces oxygen intake.

I read somewhere that it is not recommended to use the drops when climbing hills because it restricts your breathing.