On the grounds that [closed]
Solution 1:
It's possible people in the legal profession may tend to prefer the singular form where there's only a single reason/justification, but I suspect the only reason we're more likely to encounter ground in such contexts is because "legalese" is a relatively conservative context.
In more general use, plural grounds became the more common form some decades ago, and I don't believe many "ordinary" speakers would ever stop to consider how many there might be...
