The package picom-conf install the file /etc/xdg/autostart/lxqt-picom.desktop
Such file contains: OnlyShowIn=LXQt;
So when login with LXQT the command 'picom' get executed.

The package picom (since version 10) install the file:
/etc/xdg/autostart/picom.desktop
Such file does not contain any 'OnlyShowIn'
therefore when login with every desktop environment the command 'picom' get executed.

There are two problems here:
1) two files: lxqt-picom.desktop and picom.desktop starts the same program when using the LXQT desktop environment
2) if using a desktop environment that starts his own composer (for example kwin on plasma/kde) picom.desktop starts picom as well. 
At the end of the login process there are two composer running at the same time which is not good.

In my opinion the package picom should not install /etc/xdg/autostart/picom.desktop

picom and picom-conf are installed by the lxqt pattern, and since 
picom-conf will start picom on lxqt login, there is no need for
/etc/xdg/autostart/picom.desktop.

I'm going to submit a SR to remove the file /etc/xdg/autostart/picom.desktop
from the package picom.