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?
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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)