Broken Keys Repair

[Doug Renz Piano Tuning]

I tuned a piano today and the key broke where it
was previously repaired before.  It broke in the middle
near the balance rail hole.  I could see that it broke
and was glued at some point.

My question is, why do they break, and what method/glue
do you use to prevent breakage again like this after I repair
it?

Thanks!

Doug Renz
Associate PTG
Rochester, NY
pianotuner@frontiernet.net