Recall that Wilson shot and killed Michael Brown, an 18-year old black man, in August 2014. The circumstances of the shooting were controversial.
In that case, the prosecutor, Bob McCulloch was asked by activists to recuse himself from the case because of apparent biases in his personal history (his father was a St. Louis police officer who was shot and killed in the line of duty).
After the grand jury declined to indict Wilson, activists claimed that this probably happened because McCulloch convinced the grand jury not to indict.
This story is interesting because it provides evidence that in the "normal" course of operations of a grand jury, grand juries nearly always indict as long as there is something which looks like plausible evidence. So when the person in this story says "We had, as ever, no choice. We voted to indict" — that is interesting to compare against the Ferguson grand jury's decision not to indict, a very, very rare occurrence which should raise questions about how it happened.
Wikipedia glosses this as "Legal analysts raised concerns over McCulloch's unorthodox approach, asserting that this process could have influenced the grand jury to decide not to indict,[63][79] and highlighted significant differences between a typical grand jury proceeding in Missouri and Wilson's case.[33]".