S. R. Faruqi:

In the [earlier] editions of Asi and Abbasi palaa jaataa thaa is given, which is meaningless. The late Abbasi Sahib told me that one ought to read it as pilaa . This is meaningful, but the rhyme-words of the ghazal are jalaa , malaa , ;Dhalaa etc.; pilaa can't pass among them. Kalb-e Ali Khan Fa'iq has written sail-e balaa , which is not meaningless, but is also not very suitable. Kalb-e Ali Khan Fa'iq has also not told us in which manuscript he saw sail-e balaa . One possibility is that the way in Mir's time hilnaa was also pronounced halnaa , in the same way there might perhaps have been, from pilnaa , a word palnaa . But I haven't seen any such example. Thus instead of palaa or balaa , for the present I consider chalaa to be better. And then, juu;N sail chalaa jaanaa has already occurred in {667,6}. For the meaningfulness of vaaqi((ah see {104,2} and {1080,5}.

In the present verse there's a strange scene. Mir, in a state of strange distraction/dementedness, is going toward the desert. That is, some frightful thing is running after him, or he is fleeing from some terrifying event or scene or situation. But it's being said that some event 'was before' him, as though something was ahead of him as well, and he was drawing it toward himself. In the story of Hatim Ta'i this was the situation at the Koh-e Nida too-- a voice kept coming, 'O generous one! O brother!', and the hearer, losing control, would leap over a wall.

In the first line the insha'iyah -- that is, interrogative-- style is also fine. It should also be noticed that the speaker is some traveler or spectator; whatever was to happen to Mir has by this time already happened. The speaker feels more astonishment than regret. The occasion for regret is for us to feel. But the whole atmosphere is 'dastan'-like; thus in the verse there's more mystery and less melancholy.

Like one, that on a lonesome road
Doth walk in fear and dread,
And having once turned round walks on,
And turns no more his head;
Because he knows, a frightful fiend
Doth close behind him tread.

--(Coleridge, 'The Rime of the Ancient Mariner', Part VI)

FWP:

SETS == GESTURES; KIH
MOTIFS == DESERT
NAMES
TERMS == DASTAN

The versatility of kih means that several temporal and causal sequences can be possible:

=When Mir used to run madly toward the desert, what (terrifying? desirable?) thing confronted him at such times?
=What (terrifying? desirable?) thing used to confront Mir, causing him to run madly off toward the desert?
=Who even has any idea what caused Mir to run madly off toward the desert? (It could have been anything, or nothing.)

In any case, whatever it was seems to have coincided with his running toward the desert habitually [chalaa jaataa thaa], not just on a single occasion. But he doesn't do so any more. This gives the report a somewhat reminiscent feeling-- not the immediacy of a traveler's or spectator's report, but the thoughtful, meditative mood of long-ago gossip rehashed among old friends.

Moreover, the flexibility of vaaqi((ah opens up a number of possibilities from the disastrous to the neutral, from the real to the visionary (see the definition above).

It's also piquant to imagine 'Mir' running off toward the desert 'like a torrent'. Many dry deserts have infrequent but torrential rainstorms that cause deadly flash-flooding and carve out deep wadis-- and also enable all the desert flora and fauna to live. Does the lover shed a dangerous 'flood' of tears, or does he run so madly and unstoppably that he himself somehow becomes a torrent?

And in fact we don't even know whether he actually got to the desert at all. We just know that he used to run off 'in that direction' and disappear from the speaker's view.