I have seen this before, and quite a bit of the time it is caused by how the software interprets the data that is reflected back to the unit. If it was a Doppler Radar then it is even more susceptible to this kind of return.
One thing is that the software has to be written to take into account many kinds of weather returns and it also has to filter out quite a lot of
objects that are not weather related.
I could write pages on this but it would drive you to tears .
What you are seeing here is not that uncommon!