Recently I visited Srisailam, Andhra Pradesh, one of the 12 Jyotirlinga. On the way I found out that Hindus mostly there, instead of burning the dead body, they buried dead body in the graves. As far as I know, only children and sages should be buried otherwise a dead body should be burned only. Why is this difference ?
Burial of the dead is a common practice in South India. There are separate places for burial of the dead and separate for cremation. This practice is prevalent in some castes. I know that Shudras and bury their dead ones and make a memorial over the buried place. There are also many other communities which follow burial process not only for children, but also for the adults. We don't know whom you have seen in Srisailam. So, we don't know much about this case.
A pit is dug and the body is buried in that. The family, friends and relatives pour mud over the body to bid their final farewell. This is called "Bonda Pettadam" in Telugu. There is a common scolding in Telugu "Nee bonda". It is used by people when they are frustrated or irritated by someone. It means that you may be buried. Burying means burial, after the body is buried, people put mud over the body. This is called bonda pettadam.
The origin of this practice can't be traced and is unknown. But some scholars and historians say that burial is not confined to only a particular region but it was common during ancient India. They cite Rig Veda Mandala 10 Hymn 18 mantras 10-13 as a possibility of it.
The author of the book Hindu Saṁskāras: Socio-religious Study of the Hindu Sacraments
Inhumation or burial proper is almost absent in the present day Hindu funerals, except in cases of great saintly personalities and very mall children.’9 But the existence of this custom among common people in the Rigvedic times is proved by the verses contained in it. Addressing the dead body carried to and lying in the burial ground, the priest says: “Go to this thy mother, Earth, the widespread, delightful Earth; this virgin (Earth) is, as soft as wool to the liberal worshipper; may she protect thee from the proximity of Nirtti. Earth, rise, above him; oppress him not; be attentive to him and comfortable; cover him up, Earth as a mother covers her child with the skirt of her garment. May the earth heaped over him lie light; may thousands of particles (of dust) envelop him; may these mansions distil ghee for him; may they every day be an asylum to him in this world. I heap up the earth around thee placing (upon thee) this clod of earth; may I not be injured; let the Pitrus sustain this thy monument; may Yama make thee a dwelling here.’’
The author also gives some possible reasons why Sayana's interpretation on those mantras could be wrong and not valid. You may read further in the book.
Ralph T.H. Griffith translates those mantras as following.
10 Betake thee to the lap of Earth the Mother, of Earth far-spreading, very kind and gracious. Young Dame, wool-soft unto the guerdongiver, may she preserve thee from Destruction's bosom.
11 Heave thyself, Earth, nor press thee downward heavily: afford him easy access, gently tending him. Cover him, as a mother wraps her skirt about her child, O Earth.
12 Now let the heaving earth be free from motion: yea,—let a thousand clods remain above him. Be they to him a home distilling fatness, here let them ever be his place of refuge.
13 I stay the earth from thee, while over thee I place this piece of earth. May I be free from injury. Here let the Fathers keep this pillar firm for thee, and there let Yama make thee an abiding-place.
Conclusion: It is a common practice in South India to bury the dead. It is a tradition. Not all the people follow cremation.