Lets assume that we have magic users that often like to hurt each other.
Now the simplest option is to try to blast the other mage directly, but it is not without drawbacks. First the enemy will be often magically protected making our work harder. Second they will try to magick us back making the whole endeavor risky.
The idea of hurling non-magical projectile at target using magic is known - it allows to attack from distance - and avoid or overcomes many forms of magical protection if the object is big or fast enough.
Dropping object from great height is next logical step that occurred to the mages - spells to magick a mass high enough exists and are reasonably well known.
Now my question is what could prevent escalation to the extreme - orbital bombardment?
Assuming that spells to put a house sized object into orbit are available to reasonably powerful mage - and the cost of them does not increase proportionally to the destructive energy of object dropped.
To clarify more there would be a two main ways to put the payload in orbit. First one is to levitate it up - if a mage can levitate a given object at all they can move it up as high as he wants until spell runs out - but it is slow - you can only get about 3-4 kilometers in hour. Second one is to teleport it instantly - again if you can teleport object of given size and mass at all you can teleport it anywhere - and as one can teleport payload to the other side of the planet without issues we have to assume that magic somehow takes care of rotation of the planet. Both spells are reasonably common and there are a lot of mages able to perform them.
Now as the spells are common there is a very little chance that the target can detect them and know that this time they are used to make a surprise for them instead of simply moving goods and people around.