What causes housing of the starter to crack completely off? on 1994 Mazda B2300

Original starter went out approx 9 months ago 1st replacement starter lasted 7 months and 2nd replacement starter lasted 2 months both had cracked off housings that supports the end bearing of the shaft. I will say that the original starter end housing looked a little more beefed up than the replacement.

Asked by for the 1994 Mazda B2300