Why does broccoli turn brown when steaming?
Per the transcript of Alton Brown's Good Eat's episode If It Ain't Broccoli, Don't Fix It:


  Inside broccoli, nice, bright green chlorophylls are kept separate
  from acidic elements by cell walls. But if you overcook the broccoli,
  the cell walls can collapse, and the acids can attack, turning our
  nice, bright green chlorophylls into a sad, dingy gray.


This would let other pigments reveal themselves, turning making the appearance of your broccoli yellowish or brownish.  Your picture shows that the individual florets seem to be most effected, which are the smallest and easiest parts to cook on the broccoli.

So it may simply be an issue of overcooking the little florets.

Later in the transcript, Brown recommends a hybrid method of cooking, giving different treatment to the stalks and florets to minimize this effect.

According to The Happy Scientist, raising the pH with baking soda will prevent the enzymes from effecting the chlorophyll, but this would be difficult to apply when steaming, and of course it also would have the unfortunate side effect of turning the broccoli mushy.

Personally, I prefer to roast broccoli rather than steam it.   It is a little slower, but can be very effective, and of course, a different type of browning is somewhat expected :-)