They are mostly pre-arranged trades outside of trading hours, and usually not from a regular-Joe trader. Though there is technically no trading during after hours in pinks/otc, this is a settling of an arranged trade. For instance, if you were to place an order after hours electronically, it shows placed but will not execute until market open. Simplistically, it is the same with t- trades, its just a broker manually/personally contacted a MM to execute a trade at a specific price. When its filled, it will often post after/before hours.
This happens a lot when someone overseas wants to buy and is not available during our trading hours, or if a large buyer wants a chunk at a certain price, or if a large holder is unloading shares below market price and the MMs want to grab them etc.
There are many reasons why they can occur, but unfortunately the form t trade doesn't give any insight into where the shares came from. They could be from existing inventory, new inventory, a trade between market makers....whatever. No one ever really knows... lol. In fact, the MMs even have up to a year, apparently, to post the trade, so it is not necessarily even from today's trading (though it probably is within a day or so, imo). However, unless they are REALLY huge numbers below the market price I don't concern myself with it much.
FINRA's last publication on Form T reporting said: