The St. Louis Symphony has been on the rise ever since David Robertson took over as music director last year, but the amount of turnover among the orchestra’s musicians has hit a near-record high. 17% of the SLSO started work within the last two years, a very high number for an American ensemble. But there probably isn’t anything to be alarmed about: a combination of natural attrition and the sudden filling of several long-standing vacancies accounts for the influx of new blood.