By downshifting and using a lighter throttle setting to achieve the same acceleration that you had in the tall gear at low RPM, you are relieving two of the conditions that are conducive to knock:
The lighter throttle setting reduces the peak pressures and temperatures in the combustion chamber. The pocket(s) of unburned mixture that were previously prone to detonation on a hair-trigger are now more stable, and can be combusted in the normal fashion without detonating.
By operating at a higher RPM, the combustion takes less time. This is because of in-cylinder turbulence, which scales with RPM and results in stretching/distortion of the flame front that speeds up the combustion process; that RPM-scaled turbulence is the reason an engine can both idle at 600 RPM and make power at 7500 RPM. The total time required for combustion matters for detonation because of a fuel property called ignition delay. Rather than copying and pasting, I will refer you to this post of mine last summer that explains the issue in detail: