When the first Europeans came to Easter Island, the Moai were still mostly upright, but by the 19th century, all of them had been turned on their side.
Jared Diamond speculates that this is because the islanders associated the heads with the older social order which had caused the decline in their civilization, but I can't find any concrete evidence for this.
Are there any accounts from the Rapa Nui that explain who knocked the Moai over and why?
We simply don't know for sure – the Easter Islanders left no written record, and in the next century their population was so decimated by fighting, disease, and the effectively forced export of a large proportion of the surviving male population to do indentured labour in the guano mines of Peru that very little in the way of oral tradition has survived, either. So the view of the island that we have comprises what amount to a set of historical snapshots provided by widely-spaced visiting European ships. The statues were standing in the 1720s when the Dutch came by; they were all down when the British called in the 1770s.
Jared Diamond is generally regarded as a dodgy source of information about Easter Island – his use of the Rapa Nui as an example of the sort of collective societal idiocy that destroys entire ecosystems with no thought for the future is, let's say, controversial, and has been substantially challenged by more recent archaeological discoveries, which suggest deforestation was more likely caused by rats (which ate the seeds of trees) than by the human population. But the most popular theory among archaeologists and anthropologists of Easter Island today is that the statues were probably cast down in inter-tribal warfare, which itself had an unknown cause which we can only assume stemmed from some output of the underlying trends noted by archaeologists... malnutrition, a diet increasingly focused on carbohydrates, and a significant increase in the incidence of cavities in teeth, perhaps a product of lack of calcium in the local diet. It is hypothesised that all this was probably accompanied by a steep rise in female infertility, and hence a decline in population after c.1700.
Warfare, it is possible to add, probably had as one of its purposes, or consequences, efforts either to destroy the mana (broadly, spiritual power) contained in the statues, or to take the same action in despair because that mana had failed in its duty to protect the tribes who made the statues.
I covered all this in much more detail in an earlier answer, here (by me, u/mikedash, automod), which you might find interesting.