By "manual mode" do you mean that you are using the jog buttons to move the X or Y axis under motor power, or do you mean that you have released motor power and are turning manual handwheels on the ends of the ballscrews?
In short, "435 _ axis runaway" means that the CNC control thinks that the axis is powered off (perhaps because it was powered off a fraction of a second ago due to some other stall or fault); and yet the control sees that axis (encoder) moving at 120 in/min or more.
Because of the possibility that this means a shorted output FET and an uncontrollable runaway, the controller takes away its signal from the servo drive, which forces the servo drive into a fault and causes the E-stop contactor to open.
Turning manual handwheels, you could cause this to happen if you managed to turn the wheels fast enough to make 120 in/min (2 in/sec).
Do you have any problems if you are careful to only turn the handwheels slowly?
If it happens while using the jog buttons, the likely cause is that your fast jog rate (and max rate) are set too high, causing a Position Error stall a few inches into the jog move. The control will power off the axis as soon as it detects the stall. However, if the axis is still coasting at more than 120 in/min a half second later, it will trigger the "435 ... runaway" message.
Do you have any problems if you only use Slow jog mode, or Fast jog mode with the feedrate override turned down to 50% or so?
Also, how old is your control and what is the software version?