default routing issue

I have been trying to troubleshoot this issue for 2 days and i still can't come up with an answer. we have the current setup:

FG100D <->cisco 3750 <-> ASA

on the switch, we have a bunch of static routes used for voice traffic pointing to the ASA. while our default route points to the fortigate for all other traffic. This setup suddenly stopped working and we noticed that we can't even ping the fortigate from the switch. After further investigation, we found out that whenever we add a default route to either firewalls, the switch stops communicating with that firewall. Removing the default route allows us to communicate normally again with both firewalls.

Initially we thought it was because of the switch, we changed into an Aruba 2930F, and the issue persisted

On the firewall side, there are only routes pointing to our LANs and 1 default route pointing to the internet on 2 different providers

Is this a routing loop? What could possibly cause this behaviour?