Typically 5-10 minutes after the bell market makers will clear their book. Some of those prints are from earlier in the day. There's also Market on close imbalances that need to be cleared and that one block print will just be the clearing or balancing of those imbalances. Tape reading is not as black and white as watching the size going through Time and sales. You have to watch the options, and the overall sector. Stop being a newbie and learn this.