Michigan has been tapering off and declining now in most categories for 2 weeks if you look at the peak points of CLI, positivity rating, cases, hospitalizations, ICU, etc.
CLI 4/4/21
Peak positivity 4/9/21
Cases 4/9/21
Hospitalizations 4/13/21
(which means likely many of their infections were the last week of March and first week or 2 of April. Since infections are 5 to 14+ days before these numbers come to be)
They started spiking with only about 25-28% or so vaccinated, not 40%. That's on the real low end to expect them to do too much with cases unless you have high natural immunity(it did help with deaths, getting to that later). But... I would suspect that they might have less natural immunity than others like Texas since they had more strict measures throughout most of this. So Texas, even at 5% or so less vaccinations, possibly had 10-20% more natural immunity, meaning higher overall immunity. Then you add in seasonality is helping us compared to them. They were in their seasonal trend from last year, not close enough to community immunity threshold, let guard down, stopped many distancing measures, and had a spike of cases (far less deaths than the previous two spikes at the same case rate, by the way)
There is never one singular issue. IMHO, it depends on many factors. Seasonality, vaccinations and natural immunity combined, behavior of the population, and in some cases, once it gets going, it just gets going if you have a couple of these factors working against you.