If I use Tornadus' "Push Down" attack, who chooses which Pokemon to switch?
Your opponent chooses.
Source: Furious Fists FAQ (2014-08-28), TPCi Rules Team (2014-08-28)

Relevant cards

Note: All applicable cards may not be listed below.
BW2_16
PL2_64
POP9_14
XY3_86
SWSH5_16
SWSH5_16