There is a reasonably simple physiological explanation for the loss of EQ when many guys stand (it happens to me as well — BTW it also happens with flaccid hang as well). When you change positions from lying or sitting to a standing position, you translocate some of the 5.5 liters of blood in your body towards the legs and the pressure sensors (baroreceptors) in the large arteries of the upper body detect a drop in blood pressure. They send a message to the blood pressure centers in the brain telling it that blood pressure is falling. One of these centers (also known as the vasomotor center) increases the outflow of activity to the sympathetic nervous system throughout the body, increasing the release of norepinephrine and epinephrine (AKA, noradrenaline and adrenaline, respectively). This causes the contraction of the smooth muscle of blood vessels (vascular smooth muscle) leading to increasing blood pressure to accommodate the position change.
Since there is vascular smooth muscle line the venous spaces inside of the cavernosa, it too will respond to this increase in sympathetic activity as well. Contraction of this muscle could cause a change in EQ.
Why does it happen to some guys and not others? IMO, it has to do with the amount of vascular smooth muscle in their penis and this brings us back to the discussion of penis smooth muscle and its contribution (or impingement) to PE. And this also brings us back to the discussion of growers and showers and the contribution of penis smooth muscle to this. One could image that guys with a higher component the greater the response to this change in sympathetic activity.
It is not necessarily this simple. Some people have more aggressive changes in sympathetic activity when they stand that other. (one can get a reflection of this by measuring the change in heart rate when you move from lying or sitting to a standing position - this is a reflection of sympathetic nerve activity). It is also affected by your level of hydration. When you are mildly (or more) fluid down — dehydrated — your sympathetic activity changes will be greater (particularly in the morning when you wake up after not breathing all night).
There are many other factors involved in the erection process (discussed in numerous threads on this issue all over the forum), so this is not the only possibility. Waking up with morning wood and getting out of bed is a bit different from standing up with your penis in a warm space. :) So the effect of standing and other position changes can be variable and activity specific.
As I also said this affect of the sympathetic nervous system on the penis is also noticeable in the flaccid penis. The effect of sympathetic nervous system activity may be significantly less in shower than growers because I believe that the contribution of smooth muscle in the cavernosa is greater in growers than showers (by genetics). Just my opinion.
However as suggested already, this is probably not the result of a venous leak in the cavernosa. In general, a venous leak will be present in all positions and will affect EQ in all positions. In addition IMO, venous leak is an “over diagnosed” issue in this forum (just like penile venous thrombosis).