Something strange with string replacements

[gaspar . bin]

Hello,

I was just testing if I could do some things with bash and the I came across 
this:
$ tigres="Un tigre, dos tigres, tres tigres"
$ echo ${tigres//[A-Z]/[a-z]}

tt [a-z][a-z][a-z][a-z][a-z], Ale cto kkk log nfs tes tmp tst www 
[a-z][a-z][a-z][a-z][a-z][a-z], aeat home kaka lmms Mail prog temp test Clases 
kernel kfreir Mariah Música system unbind Vídeos webdav

The reply was strange, Ale, cto, kkk, log, nfs, tes... are files in the current 
directory where I'm running this.

I was just testing, not trying to convert letters or so.

My bash Version: GNU bash, versión 4.3.11(1)-release (x86_64-pc-linux-gnu)

Thanks