Lightroom likes to use reverse psychology :) It does so in numerous places but it color temperature is one of the more obvious one.
You are right that diminishing the color temperature of an image should make it more yellow. However, what Lightroom is doing is letting you specify the color of incident light to indicate how much correction is needed. As you slide the color-temperature down, you make the light cooler and Lightroom is adjusting the image to compensate by making it warmer. When you find the right settings an image which started as too cool should end up neutral.
The crop tool as the same issue which makes is counter-intuitive. If you move the drop area as with most image manipulation software, it simply follows. With Lightroom, moving the crop area left move the image contents right. It is just the converse perspective, moving a cropping region instead of the image within a cropping area, respectively.