Strange how things work

I've noticed a strange pattern that happens every single time I go through a concert prep with my chorus.  Things will be going along okay, and then suddenly, usually about a month before the concert, I'll have an absolutely terrible rehearsal.  I'll screw up notes and rhythms like I've never seen the music before.  The director spends the entire night yelling at my section, and deservedly so.  I go home resolved to spend some time during the week working on the music. 

Of course, I never wind up working on it.  I suck at working on music at home, it feels too much like homework.  Then I go into the next week's rehearsal, and it always goes amazingly well, and usually keeps going well through the concert.  I've got no idea how it happens, because I didn't do anything to make it happen.  It also happens every single concert prep, usually with the exact same timing.  

Well, last week's rehearsal was about a month before the concert.  It went...poorly.  This week's, polar opposite.  I still don't have any explanation.  The way the mind work completely eludes me.