With the Triple 7 on ground, selecting the first FD ON activates ALT pitch mode if selected MCP Altitude is within 20 feet of displayed Baro Altitude.
Some pilots have the bad habit of setting 100kts, Hdg 360 and Alt 00000 after shutdown. The electrical power up sets the MCP altitude to 10,000 (intentional design feature by Boeing?)
How the entire crew missed this during instruments scan, and during the Takeoff review and then blindly following FDs after initial rotation, if in fact this is what happened, is another question altogether....
OW