Thanks Jack.
Low battery state is something I thought about already and ruled out as the root cause: All respective sensors received fresh batteries and the problem still occurred.
Changing the affected sensors with non-affected (from other places in the house) seems like an idea, but honestly: What is the probability that four sensors (bought at different times, i.e. most likely not the same batch) start at the very same time to cause trouble because of a sensor intrinsic reason? 
Anyhow, thanks for your thoughts! Any suggestion helps. 
BTW: I am thinking, if ZHA could have problems with Hue motion as devices. But I am unsure how to narrow down the problem. I do not want to let debug logging activated for multiple days to capture the event when a device falls off the cliff.