Cracked brake spider

Found a cracked brake spider on my dads truck today. It cracked through the top bolt hole down to the s-cam hole. Rear diff on a Meritor setup. I've never seen or even heard of this before. The entire brake chamber/slack adjuster assembly would move forward on brake application. The slack was also not working properly. With no application the whole assembly would be loose.

What causes this?

My best guess would be the top bolt stretched and became loose. The bolts were definitely rusty and starting to disappear. The truck has just over 3 million KM, maybe not a bad idea to replace the four s-cam tube bolts after 15 years or so?

 

I looked at my Ken worth it is similar except it has an extra bracket to help take some of the torque. I'd imagine that makes a difference