misplaced summer Day in May for Ray (edit)

[Achebe]

Earthworms don't really want to leave the earth, as that's how they remain safe from birds etc.
But they drown in water, so they must. It's more like the shower 'forces' or 'drives' or even 'floods' them.
(Though 'lure' does imply a destination better than the others.)
"Oblige"/"obligate", while not working, would be SO Japanese.

"Early showers lure earthworms" and "early shower lures earthworms"...
It's the latter, hands down, because haiku is about specificity, immediacy, capturing a moment,
the zen aspect of living/being/existing in the present, as the past and future are illusion.

Also, in that vein, addressing a single earthworm would come closer to invoking that ideal.

Inferred remembrance:  

Thinking of earthworm singular/zen, reminds me of the old TV series "Kung Fu"
(a Western ) with David Carradine as "Kwai Chang Caine". (Yes, I looked it up.)
In flashbacks to the monastery,  Master Po, his spiritual teacher, addresses Caine as "grasshopper".

The setup event for his spiritual nickname:

   Master Po: Close your eyes. What do you hear?
   Young Caine: I hear the water, I hear the birds.
   Po: Do you hear your own heartbeat?
   Caine: No.
   Po: Do you hear the grasshopper which is at your feet?
   Caine: Old man, how is it that you hear these things?
   Po: Young man, how is it that you do not?

I thought of pointing that out too....but it sounded wankerish in my head...ha!
Ok, how about

Early showers
prise earthworms 
lilacs burst