As an 11 year resident, it is all a mess. Bottom line, it is about power and money, short and long term. Thaksin had a lot of assests frozen he wants back, but with the (rightly) highly revered King becoming increasingly frail, there are concerns about the long term stability given his influence and place in Thai society.
When it started is a matter of opinion. Ever since Thaksin got to power, he was been building a power base (relatives promoted in the Police and Army. The 2006 coup was supposed to be the last chance to stop him before the Army was his, but that may just be talk.
Right now, on face value the issue is between the poor (who Thaksin kept poor, but less poor than before) and the middle / upper classes who don't trust him and want to keep the old systems in place.
The main problem is that neither side can see the other has a point. They both do, probably more towards the yellows, but unless there is understanding there cannot be resolution.