Strange copy/paste like behaviour

Hi guys, I have a very weird issue. I’m not sure if “Software & Applications” is the right space, but maybe some software is responsible:

When I select a text and then remove it with backspace, it sort of copies the selected text, similar to Ctrl + X.
However, this text is not saved into the normal clipboard.
When I then tap my touchpad with three fingers, it copies that text back.

What is causing it to do that? And more importantly, how can I disable it?

Configure Klipper properly. It is the tool that manages the copy/paste on KDE. In your system tray, the clipboard icon.

The three finger thing equals a middle mouse button click. So by default it pastes.