with shares reaching

(Grammar Questions I Couldn't Answer)

Students were completing the gapfill in this worksheet.

1998 saw another significant (__________), with shares (_________ing) a peak of $80. 

The intended correct answer was:

1998 saw another significant increase, with shares reaching a peak of $80. 

Before class, just to make the worksheet more difficult, I deleted the ing hint from the gapfill.
And sure enough, without that hint, all of the students put the verb into the past simple instead of the Ving form.
And then they wanted to know why, and I suddenly discovered I wasn't confident in my explanation.

I ventured a guess that it was the Ving form because it was after the preposition "with"  .  The students knew the rule that the Ving form follows the preposition, but they didn't believe that this rule applied here, because there was a noun "shares" immediately following the preposition.

Having had the time to think about it, however, I want to return to my original explanation.  Even though "shares" is immediately following "with", "shares" is not the main object of the preposition.  The main point of "with" is the shares reaching.

But I thought I'd throw it out to the Internet for a second opinion.