Factors That Would Promote Eusocial Development In Mammals?
What?
Eusociality in mammals can be defined by the following criteria: 


reproductive altruism (which involves reproductive division of labor and cooperative alloparental brood care);
overlap of adult generations; 
and permanent (lifelong) philopatry (stays (or returns) at (to) breeding ground).


Why?
Burdo et al. (2000) state that eusociality in mammals is a special case of monogamy, involving prolonged pair bonding for more than one breeding period. They argue that eusociality in mole-rats evolved from a monogamous mating system where cooperative brood care was already established. A tendency for group living is considered to be an ancestral trait in some genera of mole rats. The solitary lifestyle seen in some other genera is assumed to be a derived trait.  

The aridity food-distribution hypothesis (AFDH) suggests a causal relationship between cooperative foraging for patchily distributed resources and the origin of eusociality. This theory states that limited nest sites or patchy resources leads to benefits of staying within the natal group, including reduced dispersal risk, and the possibility of inheriting the natal nest (Plowes, 2010).  

How?
In proximate terms, in eusocial mole-rats either puberty is assumed to be developmentally delayed so that under natural conditions most animals die before dispersal is triggered. If genetic relatedness among siblings is high, then philopatry would not invoke an appreciable loss of fitness, especially if the cost of dispersing is higher than staying within the natal group. High genetic relatedness is more likely in a monogamous mating system or a highly inbred population (Burdo et al., 2000). 

References
- Burda et al., Behav Ecol Sociobiol (2000); 47(5): 293–303
- Plowes, Nature Education Knowledge (1010); 3(10): 7