I'd argue that the premise of the question is incorrect - the Valar's direct interventions did, in a number of cases, lead to catastrophic results, but by no means was that actually the case in all of them.
A list of Valar interventions in Middle-Earth:
- The first war between the Valar and Melkor, aka The Battle of the Powers - This was destructive and lead to the fall of Utumno, but was not catastrophic in that collateral damage seemed to be limited. There was not the sinking of land that was seen in the War of Wrath.
- The Doom of the Noldor/the Prophecy of the North
Tears unnumbered ye shall shed; and the Valar will fence Valinor against you, and shut you out, so that not even the echo of your lamentation shall pass over the mountains. On the House of Fëanor the wrath of the Valar lieth from the West unto the uttermost East, and upon all that will follow them it shall be laid also. Their Oath shall drive them, and yet betray them, and ever snatch away the very treasures that they have sworn to pursue. To evil end shall all things turn that they begin well; and by treason of kin unto kin, and the fear of treason, shall this come to pass. The Dispossessed shall they be for ever.
While this obviously did have negative results on the Noldor, as was the intent given it was a punishment for kinslaying, this exile of the Noldor by the Valar cannot be argued to be catastrophic in that it did not cause a catastrophe. No land was destroyed nor collateral damage caused.
The second war between the Valar and Melkor (now Morgoth), aka The War of Wrath - Definitely catastrophic in every sense of the word, with the destruction and sinking of large tracts of land. However given the forces arrayed on both sides, this is more an understandable catastrophe. Their inaction had allowed Morgoth to grow ever powerful, and the forces required to capture and imprison him clashing with those defending him are so massive as to damage the earth itself.
The raising of Numenor - Not catastrophic by any means, this was intended as gift to the Edain for their part in the previous war. This was definitely a direct intervention of the Valar, as they rose the island up off the depths.
The downfall of Numenor - While definitely catastrophic, this was intended to be given it was punishment for the Numenorians' hubris in trying to reach Aman. Secondly, this was actually an action of Eru, not the Valar. The Valar were forbidden to take direct action against Men so the Valar called upon Eru who removed Aman from the world directly.
Overall there was one action (the War of Wrath) that is the catastrophe that is most commonly referred, especially since it effectively shaped the world Middle Earth was at the point of the Lord of the Rings. The sinking of Numenor, regarded as the second catastrophe, was actually an act of Eru. There are numerous counterexamples of actions by the Valar that did not result in catastrophic results.
In the case of why the War of Wrath had such drastic and catastrophic results, it is because Valar and Maiar were fighting in the bowels of the earth itself. As one can easily imagine, this could have titanic consequences - imagine tectonic plates shifting and the like, as these are powers that could raise mountains, affect weather and the like. Both the Silmarillion and HoME point towards this:
Then the sun rose, and the host of the Valar prevailed, and well-nigh all the dragons were destroyed; and all the pits of Morgoth were broken and unroofed, and the might of the Valar descended into the deeps of the earth. ... For so great was the fury of those adversaries that the northern regions of the western world were rent asunder, and the sea roared in through many chasms, and there was confusion and great noise; and rivers perished or found new paths, and the valleys were upheaved and the hills trod down; and Sirion was no more. (The Silmarillion)
The sons of the Gods wrestled with Morgoth in his dungeons and the
earth shook and all Beleriand was shattered and changed and many
perished, but Morgoth was bound. (Shaping of Middle Earth)