Sirius twinkles because it is so low to the horizon. The light from it has to travel through a lot more atmosphere which is full of dust and dirt. This is what gives it its colors. On nights when you see it as white, This is a good indication of good "seeing" conditions.

Although the dust and dirt does change the color of things, making them typically appear redder, the cause of the twinkling and color flashing is due to the light from the star passing through layers and blobs of Earth atmosphere with different light bending properties. As the light passes through this air it can be shifted by a small amount 'off-axis' if you like, and this causes a momentary change in the amount of light reaching your eye. The star appears to twinkle.

The bending can also be accompanied by the light being spread out into its component spectral colors and as this spectrum can also shift to the side, your eye picks the effect up as a flash of color.

Dust and dirt in the atmosphere isn't directly related to seeing but rather the clarity of the atmosphere, as described by the sky's "transparency".

The question SHOULD NOT be "why is Sirius twinkling so much?" It's not acting any differently than any other time.

The question should be, "what's different about our atmosphere than before?"

Now that's the million dollar question.

1. Cosmic Space Bubbles
2. The Heliosphere
3. The Sun Cycles
4. Global Warming (not by human activity but by natural ebbs and flows. Man's effect = Negligible.)
5. Massive Methane Releases (Nearing BLEVE in many areas [link to en.wikipedia.org]
6. Weather & Temperature Change (Increasing rapidly, Dramatic change.)