If you drag/drop multiple files to Avidemux from windows explorer, they normally will 'copy' (as shown in the mouse pointer).
If you have used shift to select multiple files and inadvertently keep shift held down when dragging, the mouse pointer info will change to Move (as would be within explorer windows) - this doesn't happen with other apps (it stays as Copy regardless)
The problem then is the import fails and the source files are actually deleted (and in my experience, if from SD card, irrecoverably).
Is there a drag/drop flag available to inhibit the Move process from working (you can compare dragging files to e.g. Chrome - it will always say Copy).
Thank you for catching this issue which went unnoticed because it didn't occur at least with the default file manager (Nautilus) of GNOME desktop environment on Linux, the main development platform for Avidemux.
Should be fixed by commit [Q_gui2] Overwrite proposed drop action with a safe one (https://github.com/mean00/avidemux2/commit/ac6f21cae999b7561c427bc8129c3672dd1aad24), please try a future nightly build.