As I seen 2019/10/10 written in Japan. Does that mean Japanese write it as year/day/month format and why did they write the date like that?
"2019/10/10" is not a very good example because it contains two "10's", but if you saw that in Japan, the first "10" stands for "October" and the second "10", the day of the month.
Thus, "2019/10/10" means "October 10, 2019".
So, if it were "2019/10/29", the 29 would be the day of month.
The form is "year/month/day" and not "year/day/month".
why did they write the date like that?
Because word order differs from one language to another. In Japanese, we generally go from bigger to smaller. That is to say from largest element first, followed by less large and then, least large.
The same goes for writing addresses. For instance:
is in the order of "size".
Tokyo > Shinjuku Ward > Hyakunin-cho > 9-46-3
Needless to say, that is the complete opposite of what the same address would be written in English, which will read:
9-46-3 Hyakunin-cho, Shinjuku (Ward), Tokyo
(I will not explain the number part as it is rather complicated, but think of it as a kind of street number for now.)