I haven't really read it but I'll take a stab (with psuedo code). I think NextIndex() is incrementing s.current and modding it with servers.length.

To do this, there are three operations, SET s.current to +1, and GET s.current so that it can be MODDED with servers.length.

If that SET and GET are not coordinated between threads, then a race could sour the index. For example, if two threads call this method at the same time, they could both SET the +1 before either GETs the value, then they will both get the same value +2 from where it started instead of +1 for each caller.