Did some sleuthing:
Muhammad's 400m hurdle progression from age 16 to 22 (2006-2012) was 58.75-->56.19 (2.56sec). Then in 2013 she joins Nike and drops a massive 2.36sec PR to 53.83. 2014-2015 she bounced back to her old times 55-58sec (off the sauce, injured?) Then 2015 onwards she's a consistent 53.x or 52.x runner.
For comparison, Sydney Mclaughlin ran 55.63 at age 15 in 2014, then went all the way down to 51.9 without ever taking more than a second off her personal best year by year. Could you imagine McLaughlin suddenly dropping 2.3 seconds off her 51.9 next year at age 22?
Muhammad was a 56.x to 57.x runner throughout all of college. I find it pretty far fetched that someone would all of a sudden find that much of an extra gear in the sprints at age 22 after 4yrs of top level NCAA running. The inconsistencies in 2014-2015 only sell it more to me.