Resonance in Yig oscillators - is it normal?
That looks pretty close to double the frequency. If that's the case, then this is harmonic distortion, and it is common for this type of oscillator to produce that, especially if the output power is high. In fact, it is common for spectrum analyzers to use these harmonics to receive frequencies above the range of the YTO.  

Besides, the spec on the YTO refers to the tuning range of the fundamental output of the YTO.  It is certainly possible for harmonics to fall in this range when the YTO is tuned near the low end of the range.