Labeo rohita (Rohu), a IMC are dying by opening their mouth in a well managed fish pond

stocking density is ok, liming was done, no other external symptom of disease

If all it takes for them to die is to open their mouth in the pond, I doubt very seriously if it is well managed.

What is an IMC, a Rohu and liming? What are your water tests showing you for ammonia, nitrates and nitrites? Is the pond too shallow (heats up to high temps)? Do you have plants for oxygenation? What do they look like after dying?

Even for an experienced pond management person (which I am not), you need to supply much more information and ask a definite question.

Check this out, these are diseases that Rohu are prone to getting:

http://www.fishbase.org/summary/speciess.