How does chromosome fusion get fixed in the population?
Looking at this 2001 review paper by Rieseberg: http://evolution.powernet.ru/library/Scienc9.pdf (especially Box 2), drift and inbreeding are suggested ways that chromosomal rearrangements can become fixed. These forces are strongest small, isolated populations, such as populations experiencing the founder effect.

Another model discussed in that paper is the suggestion that some centric fusions (Robertsonian translocations) lead to little or no loss of fertility in the heterozygotes.