Use of "was drowned"

As Oald says "to drown can be used as transitive verb (1 to drown someone), and it can be used as intransitive verb with no object (2 The children drowned in the lake). It is a special and idiomatic quirk of English that for intransive to drown you can find a passive form (3 The children were drowned in the lake). But this passive form is not meant as a real passive, it has the same meaning as 2.

http://www.oxfordlearnersdictionaries.com/definition/english/drown?q=drown

Actually I don't know how this curious expression came about. Etymonline mentions to be drowned, but has no explanation for the curious passive form. (German is very clear in this respect, ertrinken is intransitive with no object, ertränken is transitive.)

Obviously speakers and writers feel that 3 is a bit queer and prefer nowadays 2.