What causes red skies? 
This morning, as I was walking to work, I noticed that the skies were different. It had a reddish tint.
What causes this?
 A: Be careful about your description. One distinction which can be made is that the "sky" is not red, it's the lower faces of low-hanging clouds. 
You've certainly noticed the fact that, near the horizon, the sun looks red. This is due to Rayleigh scattering, which is also responsible for the sky being blue. 
What is happening here is the sun (out of sight from your vantage point) has had it's shorter wavelengths (especially blue, but also green and yellow) scattered away from its path. The sun is so low that its reddish light is shining up at a shallow angle onto the lower surfaces of the clouds near you. You can see that, where the sunlight is blocked from illuminating the near sides of the clouds they look grey-blue. 
A: Why clouds scatter red wavelenght?
I think it is kind of unusual clouds. 
There is two processes going: absorbtion $A$ and reflection $S$ (scatter).
I think that these type of clouds have density which is causing ratio of $S/A$ to be big enough for thin lower border so we could see that red light.
I personally think that this type of clouds consists of water particles which are "stuck" on upper bound of troposphere. Temperature there is significantly lower, but balance of pressure coming from rising flow keeps relatively big particles from lowering into usual troposphere.
