Thailand is the world’s largest producer and exporter of natural rubber.
Rubber farmers wanted the government to give them the same support that it is providing to rice farmers. Specifically, they wanted the government to offer price subsidies.
This AFP video  shows the road blockade protest set up by rubber farmers:
Bangkok Pundit analyzed the rice and rubber prices as well as the incomes of farmers in Thailand. He pointed out that rubber farmers earned more  than rice farmers:
…from an overall review of the literature it is quite clear that rice farmers have been earning less than rubber farmers for a while. With the low prices rubber farmers are facing some difficulties, but historically they have done quite well.
He also explained that natural rubber prices are affected by the rise of synthetic rubber :
…synthetic rubber is not a perfect substitute for natural rubber in all circumstances and natural rubber is not going away anytime soon, but because synthetic rubber can act as a substitute and that price of rubber is mainly dictated by the price oil, it is not so easy to influence the rubber market
There are speculations that the bold protest actions of rubber farmers were influenced by Opposition forces whose bailiwicks are located in the rubber producing provinces in southern Thailand. Political Prisoners in Thailand  wrote that:
…the South is the stronghold of the Democrat Party, Suthep Thaugsuban is a renowned and cashed-up political organizer in the South, and that having a mass protest populated by young toughs seems like a logical political tactic if trying to destabilize and blindside the elected government.
Whether instigated by the Opposition or not, the protests have grown into a formidable movement. thaiintelligentnews observed  that:
…most neutral observer now says Yingluck has a full blown political movement, the rubber farmers protest, on her hands.
Yingluck refers to Prime Minister Yingluck Shinawatra. The protests clearly succeeded in getting the attention of the government which announced a subsidy  plan a few days ago. The Cabinet has recently approved a fund of 21.2 billion baht to increase the subsidy for rubber farmers from September 2013 to March 2014. Earlier, the government urged rubber farmer groups to form their own cooperatives ; and it vowed to extend loans to rubber-based industries to increase their production levels. Meanwhile, health authorities proposed the production of Thailand-made condoms  to maximize excess rubber supply in the country.
The subsidy plan announced by the government has divided  farmer groups. There were groups which welcomed  the support package but others have rejected it. Protest actions were conducted  this weekend and some farmers have threatened to launch more rallies.