out of my 9 piggies only Honkey and Harrison do it, I have noticed they do it just before their breakfast veg or when they want their chin rubbed.
Honkey broke a tooth last year but when he was in a different cage and his tooth grew back fine, after a visit to the vets to trim the other tooth down, to the same length as the broken tooth.
Honkey and Harrison love their attention, Harrison will pop corn while having his chin rubbed, may be I have 2 attention seeking piggies.