Timeline gets corrupted, slows down and, if more than 10 clips are selected, crashes when moving videoclips with non symmetric layout into or out from audio or video tracks. Sometimes, if enough clips are selected you don't notice the timeline corruption and Kdenlive goes straight to crashing.
I assume you tested with "kdenlive-18.08-beta18-x86_64.AppImage". This is known and under development to fix it. Wait on next beta for testing this behavior.
Solved in the actual Refactoring nightly build from 16.Dec 18: https://binary-factory.kde.org/job/Kdenlive_Nightly_Appimage_Build/lastSuccessfulBuild/artifact/ We closed this bug. If it still appears in the latest version, please feel free to re-open it and update the affected version number.