Understanding "stereotyped" noise on MISO line
Measuring with a scope is an art. It is not so much if your probes are grounded but more where they are grounded. 

Noise also greatly depends on PCB layout and the drive strength. Think if it like two person side by side: You hear the one better over the other if he shouts louder. 

Back to your signals:

Circle : 
That coincides with the first data bit. Can it be that the first data bit tells the slave to read or write? If so this is is where the MISO line goes from floating to being driven. This type of behaviour is often seen with interfaces which offer a single bi-directional data line for input and output. 

Rect: Yes your signal is noisy but as said, that is often caused by a bad probe ground.I would also check if you have correct decoupling of all the chips.  

Arrow: The moment the chip select goes away your MISO output goes tri-state. Indeed what you see is the floating net being pulled-up somewhere. As there is no chip-select it does not matter where that line floats to. It could also go low or go random. 

"stereo-typed":Getting to the core of your question: That you see the same "stereo-typed" pattern is easily explained by the fact that noise in a digital PCB is not random but caused by the switching of the circuits. The CPU is running the same program section each time and thus you get the same noise pattern each time.
You might see a difference if the driver is handled by an interrupt in which case the noise pattern depends on where the SPI interrupt occurred.
Your noise can also be caused by the SPI slave device in which case the noise pattern is more likely to be always the same.