It happens, despite many of our efforts to prevent it.
Cleaning off queen cells achieves little, in fact you may be removing the replacements for the original queen when she leaves.

I remove queen "cups", the empty practice cups bees seem to build everywhere, especially along the bottom of frames. But once those cups are "loaded", that is with and egg or young larvae, I immediately try and find the queen and remove both her and 4 or 5 frames of bees and brood (minus any loaded cells) and fool the hive into thinking it
has swarmed. During May and June I check hives roughly ever 10 days to try and prevent swarming.