Because [b]jar[/b] file is the same format as [b]zip[/b] file.
You can change extension [u]jar[/u] to [u]zip[/u], and use uncompression program (winrar or unzip ...) to open it.
So it might be the ZIP/UNZIP program that change the default program for jar.
For exmaple: WinRar ....etc.