Reverb is probably the culprit
Because reverb adds echo copies of the original, the processing we do to shift the pitch can involve the copies. This could cause flanging, which can amplify some frequencies and attenuate others. Flanging can be very pronounced, depending on the amplitude of the copies. If the echo amplitude is very low, the flanging will be less noticeable.
I believe you have defined why some folks are having "wobble" and others aren't. Reverb is most often used on the vocal track, not the whole song. Thus, processing a song with vocals (main or background vocals) would tend to produce the flanging => wobbling.
Another possible cause is if there is a "Chorusing effect" added to the vocals. This will cause problems with many signal processing programs as the mutiple "images" created by adding Chorusing can interact and cause flanging.
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