An article posted on ComicBook.com November 30, 2017 features parts of an interview with Dragon Ball creator Akira Toriyama:

Saiyans are able to tap into their true potential in two ways. First, the fighters must train often, and they must be genetically blessed with something known as S-Cells. These organic markers will determine if a Saiyan can even go Super Saiyan, and the more you have the better. According to Toriyama, "“He was able to easily become Super Saiyan thanks to inheriting lots of S-Cells".

Later it's stated that

Gohan was born before Dragon Ball Z began, but Goten was born shortly after the Cell Games wrapped. Goku has activated his own S-Cells and accumulated more of them by the time Goten was born. So, as fans saw, Gohan’s younger brother was simply pre-disposed to the Super Saiyan form since birth.

Considering the time period, Vegeta would have also accumulated more S-Cells, thus putting Trunks in the same position to have accumulated more S-Cells.

An article posted on ComicBook.com November 30, 2017 features parts of an interview with Dragon Ball creator Akira Toriyama:

Saiyans are able to tap into their true potential in two ways. First, the fighters must train often, and they must be genetically blessed with something known as S-Cells. These organic markers will determine if a Saiyan can even go Super Saiyan, and the more you have the better. According to Toriyama, "“He was able to easily become Super Saiyan thanks to inheriting lots of S-Cells".

Later it's stated that

Gohan was born before Dragon Ball Z began, but Goten was born shortly after the Cell Games wrapped. Goku has activated his own S-Cells and accumulated more of them by the time Goten was born. So, as fans saw, Gohan’s younger brother was simply pre-disposed to the Super Saiyan form since birth.

Considering the time period, Vegeta would have also accumulated more S-Cells, thus putting Trunks in the same position to have accumulated more S-Cells.