I have heard two reasons behind these displays.
1. They are to rid the home of ghosts. They residents believe that these red shirts serve as some protection against widowed spirits who will harm your children.
2. They are political displays in opposition of the current government and believe that the previous election was undemocratic. United Front for Democracy Against Dictatorship : “The UDD is composed of mostly rural masses from Northeast and North Thailand, of urban lower classes from Bangkok and of some intellectuals. Although the movement seems to receive support from former Prime Minister in exile, not all UDD members support the deposed Prime Minister.”
I am still learning the details but check this google search
The second explanation seems to be more reasonable to me, but who knows. If any Thai’s are reading this and want to give me more information I’d appreciate it!