June closed in late January of 2006-07, which was the lean snow year. February was the one good snow month that year, but June did not reopen. It would not surprise me to see them put June on a 5-day-a-week schedule.
I would have to ascribe today's Mammoth announcement to economic issues. I have perhaps never seen the disparity in conditions between Mammoth and Tahoe that exists now at this time of the season. Furthermore much of SoCal skiing has melted down since that one big storm in December. Mammoth's skiing is best in the state by a huge margin at the moment.
I also recall that Mammoth's spring business fell off sharply last season, prompting a late-season debate on the Mammoth Forum about when they should close.
Since Mammoth's business is still 90% SoCal drive-up I believe that it is more sensitive to its regional economy that most resorts of its scale. It lost 1/3 of its prior peak in the early 90's recession, which was more severe in SoCal (aerospace mainly) than other regions. CA unemployment is currently higher than national, and some parts of SoCal are among the most severe real estate bust areas in the country.
I do not have that much time at the moment, but I suspect this story is getting lots of comment on the Mammoth Forum.