Is there an ענין not to pour backwards/underhanded because that is what the chevra kadisha does? If so why do we refrain from tahara-like activities?
I don’t know of any sefer that brings a source for this custom, it is possible however that it is related to “Al tiftach pe l’satan”. Although we don’t know of a source for this, nevertheless we shouldn’t be mezalzel in it, as the Rashba (Shut 1-9) says that we “shouldn’t be mezalzel in the minhagim of the old women, because there was a reason for it even though nowadays we don’ know it.this would be similar to what some poskim write regarding the custom not to walk over a child, that although we don’t know the source of this custom we still should not be mezalzel in it.
Meir Oz pg. 150, Be’er Moshe 8-36 (1).