Why can standing up fast or bending over cause a sudden COPD panic attack in which I can't catch my breath?

Increased pressure. Bending causes increased pressure in the abdomen which is in turn transmitted to the lungs causing shortness of breath. Lungs with diminished reserves like in COPD are unable to adjust to that pressure increase. The standing up part is a different story.
COPD & Panic. With copd, standing up fast further lowers oxygen supply to brain as the blood pressure drops, hence warning signal from brain by sending down more adrenaline which can trigger panic.

Related Questions

Why willstanding up fast or bending over cause a sudden COPD panic attack in which I can't catch my breath?

Two Answers. Standing up too fast causes the blood to stay in the lower parts of the body, dropping blood pressure to the brain. Dehydration makes it worse as do certain drugs. This is called orthostatic hypotension. Bending over with COPD changes the way blood and oxygen are exchanged in the lungs, like lying flat on one's back. That is called orthopnea. Breathing improves by propping up with pillows.
Absolutely. Bending over is a frequent cause of shortness of breath in COPD and can generate a panicky feeling very quickly. Any sudden movement can do the same especially if you unconsciously hold your breath for a few seconds while getting up or carrying something.
See below. Bending over will cause the abdomen to push on your lungs and prevent them from expanding fully. Standing up fast redistributes the blood in your body and your heart, which is compromised from your copd, cannot compensate for that fast enough. These are not panic attacks, this is real. You should coach yourself how to do these movements slowly, so your body can adjust.