There is also the usage "ramp down" in addition to "ramp up"
I think the definition of a ramp as an inclined slope is what is in use in both cases. If the slope inclines upwards, it's "ramping up" and signifies an increase in whatever is being measured. Similarly for "ramping down"
Etymonline for ramp also identifies it as a slope.
1778, "slope," from Fr. rampe
Note: This also says
back-formation from O.Fr. verb ramper "to climb, creep" (12c.)
which is what @Annarita's answer has
However, the usage for a creeping plant I find is not "ramped up" but just "ramped"
(of a plant) grow or climb luxuriantly: ivy ramped over the flower