It's an active area of research but there's no consensus on why this happens other than hand wavy evolutionary biology stuff. The most interesting mechanism is paleopolyploidy [1] where the whole genome of the organism is doubled due to hybridization or DNA replication errors and from that point on the duplicates start diverging. It has occurred at least once in most flowering plants and it must have happened several times with this fern. Normally after this kind of event, the genome is paired down and duplicate genes are "silenced" in a process called diploidization [2] but if there are a bunch of transposable element, they might differentiate the copies enough to keep them before the process completes.