Here's what happened: your Main file acts as source data for all the
city files. The city file references a specific cell in the Main file.
If you insert a row in the Main file when the city file is open, the
formula in the city file will change too. If the city file is not
open, the city file will not be aware that a change has been made, so
it will look in the old location.

To keep everything current, before you insert a row for Boston (to
continue your example), you would need to open the Columbus file and
all the rest of the city files that follow it in the list.

Ugh.

Fixing it will be a bit of a project, because you'll need to open each
file and make corrections. However, if you use the same type of direct
cell reference, you'll need to do that "open each file in the list"
thing every time you insert a row. A better idea would be to set up
the City files with a VLOOKUP formula, or some such, that will
dynamically review the Main file and pull the correct city data
regardless of its location in the list. This may be a blessing in
disguise, because since city names can repeat from state to state
(there are 121 towns called Springfield, more than 1 per state). A
dynamic formula like VLOOKUP, if it's set up right, will not need to
change after rows are inserted in Main.

This is bad news to deliver on a Friday, sorry 'bout that.