It's not the picnic! The picnic is lovely, the food is adorable, and the company is excellent. The weather is pleasant with no floods in sight. His siblings are present, which is never a bad thing. Nothing is overtly wrong, but Chekov is sure he'll figure out what's unsettling him if he thinks it over long enough.
Not that he's at the picnic to think, as there are much better things to do like eating, chatting, and maybe even playing a few games.]