I'm just extrapolating from past history with non-rental MW pitchers here at Texas:
O'Leary - Great freshman year, got statistically worse during her tenure, her use dropped drastically; she did have some health issues
Simpson - great freshman year (particularly in playoffs), got statistically better, but lost command; this year is a disaster - her use has declined every year in program
Czech - got statistically better each year here, but her use dropped off every year
CG - stats haven't changed much since freshman year; her use has dropped off every year
Morgan - Has gotten statistically worse, driven by this year; her use has declined dramatically
Kavan - stats almost identical to freshman year - use has gone up.
One theory I have is perhaps coach always seems to like the "shiny new" pitcher (or is perceive to do so), and the incumbents don't handle this well, leading them to press (or sulk), which doesn't help, and effectiveness and usage falls off.