I believe the understanding of the leave supporters is that somehow they have Property Rights over Brexit,made inviolable by the marginal victory in the The Referendum.Another major problem with referendums is how changes in demography and of minds can be incorporated.
I have heard a number of people saying how a second referendum on brexit would be undemocratic (as we all have) but none can explain how one referendum binds the government forever into the future even if demonstrably against the "will of the people" now, whereas a second is fundamentally undemocratic.
Their greatest fear is of a second vote robbing them as what they perceive is some sort of "Salvation" from an imagined "overlordship" on the part of the EU