Without any provided information about resistant parents and type of inoculum, it is possible to speculate only that both parents had different types of resistance controlled by distinct recessive genes.
It can be conformed by crossing it with those of resistance parents individually, and counting for gene actions. I belived this is due to the transgression for recessive alleles from either of parents.
It is possible to obtain susceptible Fi if the genetic constitution of one of the parents is heterozygote for the resistance and the other one is made of homozygous recessive for resistance. In a nutshell, the two parents may likely have these aforementioned genotypes for resistsnce.