Serial port locked by random process

I have an issue with trying to open a USB device's serial port with pyserial. It seems that as fast as I connect the device, /dev/ttyACM0 is used by some random process.

ModemMana  851        root 8u  CHR  166,0  0t0  16925693 /dev/ttyACM0
gmain      851   879  root 8u  CHR  166,0  0t0  16925693 /dev/ttyACM0
gdbus      851   884  root 8u  CHR  166,0  0t0  16925693 /dev/ttyACM0

If I wait a few seconds it works fine. This however forces me to have a 10 sec timeout on my application which is real bummer. What is ModemMana, gmain and gdbus doing with the serial port?? Is this normal behaviour?