This time of year they're on the lookout for 8-12 similar-value payments from different people all within a short time frame. And if one of them puts anything related to fantasy or football in the description then it's another red flag. Payouts are easier to disguise.
I got flagged for this a decade ago on PayPal (who own Venmo) for a March Madness pool. They saw all the buy-ins, froze the funds and then warned me about using their platform for gambling. I had the ability to proceed anyway as long as I denied the allegation, because they believe they've taken care of their gambling liability via the warning.
All my Venmo transactions are set to private as the default, so I don't show up on anyone's feed. Trust is another thing, though.