Does low-pass filtering a square wave cause a 'ringing' to develop in the signal?
These ringing artifacts are due to the Gibbs phenomenon. They are indeed caused by lowpass filtering a function with discontinuities. What you see is basically the oscillations of the step response of the lowpass filter. The oscillations can be reduced (or completely eliminated) by choosing a lowpass filter with less or no ringing in its step response. E.g. a Gaussian filter has no ringing at all. However, lowpass filters with well-behaved step responses perform worse in the frequency domain in terms of passband deviation, transition band roll-off, and stopband attenuation.