En-spaces are non-breaking and I suspect that's where the problem lies. I fyou look closely you'll also see that the lists break between the number and the following comma in several places.
Do you have any keep options applied to the paragraph style for the listings? My guess is you may have it set to keep the last 2 or 3 lines together, but noting for first lines. You might want to change that to keep all lines together if you don't want a listing to break across columns.
Thanks, Peter. I will go back to just a normal space. In the index generate panel, they display all the white space characters as options for word & number separators. It would help occasional users like me if the documentation talked about separator characters and which ones were non breaking. Yes, I did notice the breaks between commas and in the middle of page numbers. ID does strange things when doing its index psuedo hyphenation. A nice feature would be an option on the index generation page that said "allow breaks on whitespace characters".
I'm not sure where you'll find it in the documentation, but all fixed-width spaces are non-breaking.