MOSI pin state between bytes

I'm playing around with some SPI transfers and happen to be sending all 0's from the micro, between each byte (CS still held low) I see the MOSI pin output high, while this isn't presenting any issues, I was a little surprised to see it.
This is a copy/paste of the piece of code I'm using.

            SPI.beginTransaction(SPISettings(42000000, MSBFIRST, SPI_MODE0));
            digitalWrite(FPGA_SPI_CS, LOW);
            //delay 3 us
            SPI.transfer (fpgaTxRxBuffer, sizeof(fpgaPollData));
            digitalWrite(FPGA_SPI_CS, HIGH);

The buffer is full of 0's and I happen to be using pin 87 for /CS

I've had a look at the datasheet and had a google about, but nothing seems to mention it.

Anyone got any thoughts?