I would say this is more of a stylistic decision based on branding as oppose to a UX decision.
Majority of systems do capitalize, or use title case on buttons labels. This may help in making button labels stand out as stand alone actions that aren't full sentences (e.g. Ask Question, Edit Details, Save Changes).
If your button labels are longer and more descriptive though, capital/title case may impair readability of the item.
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When you're reading the above text, the last item with all text in lower case is the faster scan because the capitalized letters can get in the way.
In the systems I work with, we have a mix of short and longer button labels. So we decided to go with title case in order to balance these two things.