It is more complicated than that. Context would be helpful.
I have seen this form several times only now to ask myself:
why "a lowering of demands" and not "lowering of demands"?
Lowering is uncountable, so it should not be given an article - that's the logic I know.
a drop in oli prices should lead to "a lowering of intrest rates"
Let me guess: is it that lowering here stands for something like reduction - despite its gerundive apparition?
To clarify: did you mean that even if we treat loweing as reduction, indefinite article is not obligatory, but optional?
If you consider today's market reaction to be one lowering and tomorrow's market reaction to be another lowering, then they are different, countable events. There was a lowering today and we expect a lowering tomorrow.