Single MOSFET motor driver with oscillation on output
It would be more illuminating if you trigger your oscilloscope off of FET gate -- that should be a cleaner signal.

What's happening is that the circuit will push current into the motor, but it won't pull it out.  So when the FET is on, if the motor is spinning any slower than it's no-load speed with 5V applied, the motor will speed up.  When the FET is off, the motor will coast along, generating a voltage proportional to its speed.  Because the system is lossy, the motor isn't spinning quite fast enough to generate 5V -- but it's pretty close to it.  You're reading the average of 5V and whatever the motor generates as it's coasting.