The intermediate levels of a factor tree can be any factors of the number at the top. The leaves are always prime numbers. The number of branches at any level need not be limited to two. This tree could have been shown as one with three leaves at the first level below the top: 2, 5, 5.
It is convenient to first choose factors that you know. In this case, we know 50 is divisible by 2 (because it is even), so we choose that as a factor at the first level.