Normally this is triggered by making adjustments to sales summaries, but it sounds like you imported and everything was entered "one by one" at detail level. few things to try:
if you have multiple 1099s, on desktop you can go into Forms mode, on the 1099-B worksheets at the bottom there is a checkbox whether that 1099-B triggered a mail-in to help narrow it down.
on the 8949s check if any items have "see attached statement" in the description.
if you had any non-covered categories this may also be triggering the mail-in tho I have avoided this with input as one-by-one; maybe an oddity of the import, a cost basis is missing instead of 0 etc, tho I'd expect that to get flagged for review
