What condition(s) would likely exist which could cause 2 chromosomes in the newly fertilized egg of a placental mammal to fuse? Assume this is more likely than having the same chromosome pair fuse in an individual egg, as well as an individual sperm.
Chromosome fusions (as well as fissions and other changes) usually take place during cell division or pairing, therefore, it's very unlikely that two chromosomes will fuse in an already fertilized egg. If they do such, this egg would probably be aborted.
One factor that may lead to chromosome fusion is loss of thelomeric sequences (telomere = chromosome extremity) (O'Sullivan and Karsleder, 2010) - but probably not in an fertilized egg.