What is the mechanism behind plants producing seedless fruits?

seedless fruit has nothing to do with being genetically modified. There are no frankenstein genes here. Over lots of generations plants with smaller and smaller seeds were chosen to produce the next generation until seeds are either not even present or very small.

These plants are reproduced by a method called splicing. This involves taking a branch of a previously existing tree and cutting a groove into a different tree and taping the new branch into it. The trees don't even have to be the same kind for the new branch to attach itself and continue growing. This is very similar to cloning the old plant.

Two plants with different numbers of chromosomes (either same or different species) are cross pollinated, the seeds from the offspring are then planted and the fruit of the resultant plants is sterile, sort of like a horse crossed with a donkey produces a mule (which is infertile).

Other plants produce seedless fruit if pollination did not occur. Yet others are just mutations.

"Seedless oranges and seedless grapes are the result of cultivation of naturally occurring seedless plants. The navel orange is descended from a seedless orange tree found on a plantation in Brazil in the nineteenth century. This tree was a mutation...Seedless watermelons are sterile, that is seedless, because they have three sets of chromosomes. This condition is called triploid." - http://www.enotes.com/how-products-encyclopedia/se...

"Parthenocarpy occasionally occurs as a mutation in nature, but it is usually considered a defect, as the plant can no longer sexually reproduce, but may propagate by asexual means" - http://www.sciencedaily.com/articles/p/parthenocar...

"Fruits that develop parthenocarpically are typically seedless." - http://waynesword.palomar.edu/hybrids1.htm#parthen...

Google it. Briefly, it is the process of cutting a branch of the fruit tree / plant and replanting. Also known as genetically modified foods.