One of my PCs grabbed someone with a garrote and was knocked prone shortly afterwards. What would happen to the grabbed target? Would he be let go (it was a human), would he get knocked prone as well (being grabbed) or something completely different?
Nothing would happen.
The grabbed condition ends when
The attacker (or the target) going prone does not meet any of these clauses and would have no effect directly.
It may seem "realistic" to overrule this, especially with a garrote, but that would be a house rule and could lead to unintended consequences. Considering that dropping prone is a minor action, this would allow the attacker to cause the target to go prone for just a minor action with no attack roll required.