Who knows what happened? Maybe Mr. Milling gave notice to take a similar job at GEL's soda ash operation and the company said leave tomorrow instead.
But there's kind of a pattern here. The CFO resigned in December, giving exactly 2 weeks notice. For a named executive, 2 weeks is also really short and for a CFO to resign like that just before the year-end close is really unusual.
It could be as simple as dissatisfaction with pay - I think CINR cut back on incentive comp in 2018 and that could be it.
I don't own CINR but it's tempting. If they were telling the truth - operations should be good this year and they cut the distribution specifically to fund the big expansion/efficiency project, maybe today's price is great. I don't see any hurry, though.