They were negative by default but they should've been positive:
> a value ranging from -32768 to 32767. Triggers, however, range from 0 to 32767 (they never return a negative value).
https://wiki.libsdl.org/SDL_GameControllerGetAxis
This fixes LTRIGGER and RTRIGGER (L2/Zl and R2/Zr on most gamepads)