Answer:
When we talk about the inheritance of traits, or the passage of traits from parents to future generations down the line, we are not just talking about the visual (phenotypical) expression of those traits, but also, their underlying explanation, which is the genotype. A genotype is basically how the genes of the parents combine in such a way that the children inherit a set of traits from the parents, and express them phenotypically, or not.
In the case of blood types, we have four phenotypic groups: A, B and O. Each one of these types is characterized by the underlying set of genes that are responsible for what is expressed. While the O blood type presents a genotype ii, which is recessive, the A and B types will have the following genetic patterns: Ia Ia, or, Ia i (characteristic of the O genetic material) for the A type and: Ib Ib, or Ib i, for the B type. When there is a genetic conjugation from parents genetic material, regarding blood type, we would have these sets of genes combining. In most of the possible combinations genetically speaking, we have the recessive i gene appearing, including in the A and B dominant blood types. This means that when crossed, there will always be a chance of at least one offspring presenting the O blood type, even if one of the parents is dominant A, or B.
In answer: it is the fact that all three types present the recessive allele i, typical of the O blood type, that when pairings of genes happen between parents, the genetic characteristic of the O type may present itself in a dominant fashion, instead of the usual recessive pattern.