Sign of respect to Sr Monk? I've seen a video where the Sr Monk's feet were being washed by laity or even Jr Monk.
Anybody know what purpose does it serve?
I think these practices crept into buddhism from local customs ,I doubt if buddhism is practised entirely in its true form in any country. As far as i know even while bathing in a river a junior monk should keep a certain distance  from senior monks and should bath further downstream.