I HAPPEN TO GET THIS INFO FROM A FB PROFILE.
During winters, frost is a major problem. When the front panto is raised, it looks like this.. ">".. Now, any frost on the wires will be brushed aside by the panto in the forward direction, and the frost drops/particles will land outside the loco. However, if the rear panto is raised, it looks like "<", and here the frost particles will get deposited on the roof of the loco, which contains DBR and other valuable equipment. There is a risk of them getting damaged due to the frost deposited by the brushing action of the rear panto. For this reason, front panto is used in winter. However, this by no means...
more... is a convincing explanation, and it cannot explain why front panto is not used in rainy season. This is a subject of intense debate, and we have not found any satisfactory explanation from even 2-3 loco pilots, with whom we have spoken. In fact, one may spot electric locos with rear pantos also in winter, though that's quite rare.