The reason I've seen stated previously is that it provided just a little bit more physical protection to the driver.
Possibly, but might also have simply been easier under some circumstances.
Locomotive already attached to wagons, but brake tender attached en-route, no need to uncouple wagons.
Whilst having the brake tender in front of the engine would seem a bit safer in case of accident, the trades unions were opposed to this and at times "blacked" trains with the brake tender in front of the engine.
I have heard, but can not substantiate, that the first brake tender had a flat top, but that this give rise to concerns about misuse by persons riding on the tender, or placing cycles etc. thereon. Curved tops were therefore used to deter such misuse.
IIRC the brake tenders were modified from old steam loco tenders, and filled with scrap iron and concrete.