Somatic cell hybridization ; How chromosomes are lost?

Interesting question, the fusion of somatic cells usually results in polyploidy, but like you mentioned, there is chromosome loss. The reason for this is primarily due to differences in generation time, for instance, a monkey will have a longer generation time than a mouse. Interestingly, in the case of the human-Chinese hamster hybrid, human chromosomes are lost. If a mutant hamster cell with a nutritional deficiency (e.g., one that cannot grow in the absence of glycine) is hybridized with a human cell without the deficiency, the hybrid cell will not show the deficiency (in this instance the hybrid will be able to grow in a medium without glycine). The hybrid cells will then produce clones showing extensive loss of the human chromosomes. The one human chromosome the hybrid cannot lose is the one that carries the human gene that compensates for the hamster cell mutation, for without that gene the cell would die. The exact mechanism why chromosome loss occurs by somatic cell hybrids is not understood, really.

Your answer lies in this paper, thanks Duddy.

X chromosome-induced reversion of chromosome segregation in mouse/Chinese hamster somatic cell hybrids : Cellular recognition of native and foreign X chromosomes