Help! Downshift paddle activate horn!

[Nismo502]

I have a e90 323i automatic and I have a very weird problem.

Every time I downshift using the gearshifter or paddle shifter, the horn goes off.
It was working fine until I sent it to replace my stereo head unit with a pioneer one.

It is as if I can activate the horn using my downshift paddle.

Do you know why? Please help.