Why is 14..Qd8 the most popular move in this variation of the Caro Kann? According to chessbase, 14..Qd8 is by far the most popular continuation in this position (196 games), even though Stockfish 8 seems to prefer 14..Ngf6 (1 game). 

I'm struggling to understand the rationale for 14..Qd8, as the queen looks reasonable on a5 for the time being (pinning the pawn on c3, which may allow tactics on d4), while 14..Ngf6 develops a critical piece which allows the black king to castle. Admittedly, after 14..Ngf6, the white attack on g7 can build up quickly with ideas of exchanging knights and then Qg4 and Rh3-g3, but black has defensive resources to deal with it and I fail to see how 14..Qd8 helps in any case.

[FEN ""]

1. e4 c6 2. d4 d5 3. Nc3 dxe4 4. Nxe4 Bf5 5. Ng3 Bg6 6. h4 h6 7. Nf3 Nd7 
8. h5 Bh7 9. Bd3 Bxd3 10. Qxd3 e6 11. Bf4 Qa5+ 12. Bd2 Bb4 13. c3 Be7 
14. Bf4 Qd8!? {+0.22 - most popular move}  
( 14... Ngf6 { -0.14: Engine likes this better} )
This is ctg format property, or bug, if you like. There were no games played with 14.Bf4 and as well there were no games played with 14...Qd8. The ctg tree just knows position after 14...Qd8. Certainly it happens via different move order. So in 196 games wasn't played 14...Qd8, but 196 games saw the position arising after 14...Qd8.

This position arises in 11...Bb4+ 12.c3 Be7 line after move 12.

I dislike this ctg thing as it can hide a lot of refutations in move order sensitive lines. It navigates you towards most played positions, it doesn't show you the most played moves.