Villages will oven generate over hills, mountains, and other unfriendly terrain that no reasonable person would ever build their home in. This happens often (though not exclusively) when a village bleeds into another biome, but I have reported that issue in a separate bug report. This happens to every type of village, though it is most frequent in taiga villages, as they have the most mountainous terrain. This bug could either be seen as villages generating where they shouldn't, or villages not generating properly to adapt the terrain at hand. 

Reproduction is easy, as this bug is extremely common. Simply go a world and keep finding villages; more often than not they will have the same issue that i refer to here.