Did you manage to work out what was causing this? I'm also measuring very similar negative voltage spikes with a picoscope 2405a, from several different sources and using different laptops, which makes me think I have a problem with the picoscope itself.

Hello, I've been trying to diagnose some issues using a picoscope 2405A, but keep seeing some strange intermittent signals. Attached is the data file. I've seen this same stepped signal at approximately 2MHz frequency in all my measurements, the attached data file was found when attached to a 18V wa...