Hello Jamil. Can you let me know what type of heat gun you are using? Is it something like this or this?
There is an important distinction here. The melting point for lead-free (SAC) solder used in iPhone logic boards is ~217C, depending on the actual alloy. However I suspect the solder used for the shields is a different alloy with a slightly lower melting point so that when they wave solder the shields they don't melt the solder for the components.
At any rate, that means that when you apply heat with forced air, your goal is to get the shield solder joints up to the melting point. You can't just put your gun or station at ~215C; too much heat is lost via air convection and the rest of the logic board also acts as a heat sink. Now if you set a heat gun to 350C and force too much air for too long, you will eventually get the board too hot and the solder balls underneath the IC will melt and blow out, causing shorts and disconnections.
A hot air station set to the same temperature will throw a lot less air and it will be more localized. You still have to be careful about transferring too much heat but the process will be slower and give you more time to accomplish your tasks.
Either way, the secret to successful use of hot air on an iPhone logic board is getting the right amount of heat transferred. Too much and you can kill your board...which is most likely what happened here.