It's not always a scumtell. There are situations where town can, via burden of proficiency, identify scum that are voting them, knowing that the player voting them SHOULD be town.
Most situations are where the player thinks it should be the above, but are just mistaken, overestimating their townness and/or the competency of the player that they are omgusing.
However. There are cases where OMGUS, instead of coming from town, comes from scum. And these are the cases where a player shows no scumread of a player, or if anything show signs of defending that player, but when that player scumreads them and presents decent reasoning, the scum player realizes that they need to fight back. Arguing that they are town is usually not a good option, so instead, their stance shifts into scumreading the player that they previously were not.
These are cases where the aim of the scum player is not to lynch the player in question, but rather discredit them. Failing that, to convince players that the fight is TvT (well, close enough in this case).
That is the basic difference between town omgus and scum omgus. The aim of town omgus is to lynch the player that they are omgusing on the basis of believing (quite often mistakenly) that the push on them came from scum.
The aim of scum omgus is to save the life of the scum player.