Apparently the Tory-LibDem coalition violates a lot of our theoretic understanding of the coalition-formation process. I see two factors in play here. One is that our understanding of this subject comes from countries that habitually feature coalition governments, of which the UK is not one. The other is that the ideological identification of the Liberal Democrats is contested and a bit problematic. They appear to most outside observers to be a center-left party that’s in many ways to the left of Labour, but their traditional location is as a centrist party to the right of Labour and many Labour supporters also see it that way.