To include headers from other headers in the same library, we should use #include "...". This is a common manner (examples: 1, 2, 3).

However, the current AtCoder Library uses #include <...> (e.g. https://github.com/atcoder/ac-library/blob/114e690ade7fe839db3ea0e5f169207672ef0886/atcoder/all).
#include <...> is for headers in the standard system directories (see https://gcc.gnu.org/onlinedocs/cpp/Search-Path.html), but AtCoder Library is not system's one.

IIUC, if you use #include <...>, a problem will happen when multiple versions of the this library are installed.
For example, let say we have /usr/include/atcoder/segtree.hpp, /home/user/kimiyuki/GitHub/ac-library/atcoder/segtree.hpp, and /home/user/kimiyuki/GitHub/ac-library/atcoder/all. Under such situation, even when you include /home/user/kimiyuki/GitHub/ac-library/atcoder/all with #include "atcoder/all", this atcoder/all ignores /home/user/kimiyuki/GitHub/ac-library/atcoder/segtree.hpp and includes /usr/include/atcoder/segtree.hpp. This will cause mysterious bugs.