How do I explain my poor grade to a grad school committee?
I've been a straight A student before university all my life.


Great!  You're finally among your peers, who have also been straight-A students their entire life before university.  Finally, schoolwork isn't just a trivial boring exercise in pencil-pushing!  Finally, you can actually test your limits!  Finally, an opportunity to be challenged, to be confused and frustrated---in short, to actually learn something! 

By definition, only a small fraction of your class can be significantly above average.  Fortunately, graduate admissions committee know this.  We don't just look at your grades; we also look at the institution that gives them.  A perfect GPA from a school that typically admits weaker students is worth significantly less than a merely-pretty-good GPA from a school known for admitting strong students.  A 3.2 from MIT or Harvey Mudd is worth more than a 4.0 from the University of Southern North Dakota at Hoople.

Do not under any circumstances make excuses for your grades in your graduate school application.  Write about your accomplishments.  Write about your expertise.  Write about the challenges that you overcame.  Present the evidence that you will thrive in your target graduate program(s).  Sell yourself as a mature, hard-working student, ready to hit the ground running.  Do. Not. Make. Excuses.  No matter how unfairly you believe you have been treated, admissions committees will read your complaints as whining, and they will admit some other student with a mediocre GPA from a hyper-competitive school instead of you.