It's not from the scripts, to put it very simply, when the second or later person clicks on the signature
field they are changing the form data, which is exactly what the warning says.
E.g. Form with 2 signature boxes, BoxA, BoxB.
Person 1 signs BoxA, this stores the contents of the form, including BoxB which is empty.
Person 2 then signs BoxB, which stores the contents of the form at signing time.
When the document is opened the contents of the form now match the contents stored when BoxB was signed, but for BoxA the content is different (there is now data in what was previously a blank field) so the warning is triggered.
The warning firing is the correct behavior, as if there was no warning then person A could sign a particular form which person B could then change the contents of without their knowledge, and it would still appears as though they signed it. If the previous version was not stored, and a warning not issued, then signing a document would be pointless.
You can make certain fields be locked after signing, and this can be handy as if you lock all the fields which should not be changeable after the first signature, then you can be pretty sure that the data hasn't changed, and the warning can be safely ignored (but as far as I'm aware, not disabled).