Scrambling
Definition: In languages with free word order, the phenomenon of the words in a clause being able to be arranged in (apparently) any order.
Japanese is said to be a SOV language. English on the other hand is said to be a SVO language. English is quite strict in terms of how constituents are arranged, but Japanese has relatively free word order with the only restriction being that the verb must come at the end of a clause/sentence. Japanese is more restricted when it comes to scrambling compared to other lanaguges which allow it freely like Turkish.
Some of you may have encountered sentences where a noun phrase follows the verb in Japanese. This usually happens in the spoken language and this phenomenon is known as "Right-dislocation".
e.g. トムを殺した、ボブが
When this happens the noun phrase after the verb is considered to be an afterthought.
Now back to scrambling. Consider the verb くれる, a verb of giving done by someone other than the speaker. This verb needs three arguments, noun phrases needed to complete its meaning.
1)A person who receives the item (Indirect object)
2)A person who gives the item (Subject)
3)The item to be given (Object)
Take the following example
1)トムが私に食べ物をくれた
Tom gave me food
Since Japanese allows scrambling, provided the verb is at the end, we have 6 possible ways which we can order the example above.
2)私にトムが食べ物をくれた
3)食べ物を私にトムがくれた
4)トムが食べ物を私にくれた
5)食べ物をトムが私にくれた
6)私に食べ物をトムがくれた
All of these sentence mean the same, but not all of them are natural.
1,2,5 and 4 are all natural, but 3 and 6 are not.
If we consider scrambling to be a fronting operation and that in order for a sentence to be natural, only one constituent can be fronted.
2,5 and 4 are all natural because only one constituent is fronted.
In 2 私に is fronted, placed before the subject, and since only one constituent, excluding the subject, is fronted, it is natural and well-formed.
The same applies to 5 and 4 where only one constituent is fronted.
However, in 3 and 6, two constituents are fronted, two elements are placed before the subject.
In 3 食べ物を and 私に are placed before the subject トムが.
For more read "The Langauges of Japan" (1990)