Sources from Nehama Leibowitz' introduction to Exodus:
1. ...When Joseph died, they abrogated the rite of circumcision. They said, Let us be like the Egyptians. Because they did so, the Holy One Blessed be God turned the love which the Egyptians bore them into hatred.(Shemot Rabbah)
2. 'And the land was filled with them' - the amphitheaters and circuses were full of them.(Yalkut Shimoni)
3. The Israelites were punished for violating Jacob's wish that they should live apart from the Egyptians in Goshen.
4. [There is] and educational motive for the exile - so that we observe the commandments as a result of remembering how we were once slaves and unable to worship God freely.
Encyclopedia Judaica "Slavery":
BIBLICAL LAW
The Hebrew term for slave, 'eved (pl. 'avadim), is a direct derivation from the verb ʿbd, "to work"; thus, the "slave" is only a worker or servant. The eved differs from the hired worker (sakhir) in three respects: he receives no wages for his work; he is a member of his master's household (cf. Gen. 24:2; Lev. 22:11; and see below); and his master exercises patria potestas over him; for example, the master may choose a wife for the slave and retains ownership of her (Ex. 21:4) and he has proprietary rights in him (see below).
Classification
The following classes of 'avadim are to be distinguished:
HEBREW SLAVES
A Hebrew could not become a slave unless by order of the court (for which see under Criminals, below) or by giving himself voluntarily into bondage (for which see under Paupers, etc., below; Yad, Avadim 1:1). Other slaves were always recruited from outside the nation. It has been opined that the epithet "'eved 'ivri," and the laws relating to Hebrew slaves (Ex. 21:2–6) would apply also to such non-Jewish slaves as were born into the household as the offspring of alien slaves (see, for instance, Saalschuetz, Das Mosaische Recht (1853), ch. 101).
ALIEN SLAVES
"Of the nations that are round about you, of them shall ye buy bondmen and bondwomen. Moreover of the children of the strangers that do sojourn among you, of them may ye buy and of their families that are with you which they have begotten in your land; and they may be your possession" (Lev. 25:44–45).
PAUPERS AND DEBTORS
A debtor who is unable to pay his debts may give himself in bondage to his creditor (cf. Lev. 25:39; Prov. 22:7; see also II Kings 4:1; Isa. 50:1; Amos 2:6, 8:6; Neh. 5:5). According to other opinions, the verse in Leviticus 25:39 deals with an ordinary pauper who sold himself and the debtor's bondage was against strict law, although it happened from time to time in practice (see Elon, Ḥerut ha-Perat, 1–10, and n. 9; *Execution (Civil)).
CRIMINALS
A thief who is unable to make restitution is "sold for his theft" (Ex. 22:2).
PRISONERS OF WAR
It would appear from Numbers 31:26–27 and Deuteronomy 20:10–11 that prisoners of war could be, and were, taken into bondage, but it has been contended that no prisoners of war were ever taken into private slavery (Kaufmann, Y., Toledot 1 (1937), 651).
FEMALE SLAVES
A father may sell his daughter into slavery (Ex. 21:7), usually apparently for household duties and eventual marriage (Ex. 21:7–11).
CHILDREN OF SLAVES
The Bible mentions "the son of thy handmaid" (Ex. 23:12), "he that is born in the house" (Gen. 17:12, 13; Lev. 22:11), indicating that the status of slaves devolved upon their children.
EJ:
Treatment of Slaves
In the case of a pauper who sells himself into slavery or a man who is redeemed from bondage to a stranger, no distinction may be made between a slave and a hired laborer (Lev. 25:40, 53). A master may not rule ruthlessly over these slaves (Lev. 25:43, 46, 53) nor ill-treat them (Deut. 23:17); Ben Sira adds: "If thou treat him ill and he proceeds to run away, in what way shalt thou find him?" (Ecclus. 33:31). A master may chastise his slave to a reasonable extent (Ecclus. 33:26) but not wound him (Ex. 21:26–27). The workload of a slave should never exceed his physical strength (Ecclus. 33:28–29). A fugitive slave must not be turned over to his master but given refuge (Deut. 23:16). There was no similar rule prevailing in neighboring countries (cf. I Kings 2:39–40). The *abduction of a person for sale into bondage is a capital offense (Ex. 21:16; Deut. 24:7). In general, "thou shalt remember that thou wast a bondman in the land of Egypt" (Deut. 15:15), and that you are now the slaves of God Who redeemed you from Egypt (Lev. 25:55).