Reasons could be things like a death in the family, women having their period.
Does this come from scripture and is it practiced outside the South today?
Those periods, when impurities prevail due to birth or death that occurred in a family, are called the Asaucha period and it is a scriptural thing so all Hindus irrespective of the region they belong to must follow those rules.
Birth/death related impurities are called Jataka/Mritaka Asaucha.
Now I shall speak of the term of impurity resulting on the occurrence of a birth, or a death. Brahmans in three days become pure again, when a relative has died, or has been born.
A Kshatriya recovers purity in twelve days ; a Vaisya, when fifteen days have passed ; a Shoodra, after the lapse of a month ; this is conformable to what Parasara has said.
Parashara Smriti, 3rd Chapter
Asaucha is the word and it comes of Asuchi which means impurity :http://sanskritdictionary.com/?iencoding=iast&q=अशुचि&lang=sans&action=Search
In the sraddha-mantra, it is mentioned that the sraddha has to be done "asauchantyaat dwitiye ahni" meaning on the second day of ending of Asaucha. So Asaucha is a must for sraddha karma in hindu culture.
As Rickross points out,the period of Asaucha varies from castse to caste. Many mentioning a comment of Sataatapa, opine that ten days asauchaa is precribed by Sataatapa.But this prescription is not found in Satatapa-Smriti, as per Sri Sitaramdas Omkarnath(Omkarnath Rachanavali, vol 7)