There is a bug where the traffic on VLAN links is reported as twice as much:
https://redmine.pfsense.org/issues/3314
Separately, the WAN links where you are using your OpenVPN across may increase in latency as traffic increases.
This causes the gateway monitoring agent to react adversely.
I've had some success in increasing the latency monitoring thresholds in those links:
System | Routing
Edit the respective gateway, press 'Advanced'
Change the latency thresholds (I used 750ms/1000ms)
This was definitely helpful
However, there may be a bug in play:
@cmb:
If you don't have any gateway alarms, there is a potential second cause I just fixed. Edit /etc/rc.newwanip and find where it has curnwanip in there. Replace that with curwanip (just remove the n), save, might want to reboot afterwards just to be 100% sure nothing is using the old code. Be careful when editing any code like that. That made things think your WAN IP had changed in cases where it hadn't, so it did things like restart VPNs where it was unnecessary.
I'm going to revert the latency settings to default and test the /etc/rc.newwanip fix tonight.
See this thread for more info:
https://forum.pfsense.org/index.php?topic=76735.0;topicseen
-nb