To be honest, with how crowded summer is starting to get, it makes sense that films like Jumanji would move to earlier or later points of the year, to flesh out the yearly Roundup of films.
Heck, films like Get Out, Deadpool, and Lego Movie are exceptional examples of releasing films like those out of the norm, as it would drive theater sales.
Edit: and Jumanji wasn't released this month. It was released last month, one week after TLJ.