The spiderlings attack the champion that has a weak affinity. for example if the spiderlings are force  ( red ) they will target magic champions first ( blue ) and so on.. 

As far as Imkjow, that is not entirely correct. The stage 20 Spiderlings like to gank my Void affinity Coldheart and ignore my weak Force affinity Drexthar Bloodtwin.

Good that Drexthar and Martyr both have an AoE Provoke.

I saw somewhere that the lowest HP gets focused on a lot too, which would explain a Void champion getting attacked.

Yes, low hp and/or low hp ratio is also a factor