Yes, odds are that more than just the WFCO converter/charger are on that circuit breaker, so you need to figure what set of things share it and which of them (or combination of them) is causing the breaker to trip. The breaker responds to the total load, so it could be an intermittent short in one thing, or the combined amp load of several things.
If the batteries are fairly low on charge when you plug in, the WFCO's charger would kick in at a relatively high amp rate. If there are other things sharing that circuit that are plugged in & turned on, it might exceed the breaker capacity. Things like electric space heaters, hair dryers, coffee makers, etc, all have a fairly high amp draw. I mention the WFCO power draw specifically because it has a fairly broad range depending on the circumstances, so that could explain why sometimes the breaker trips and other times it does not. That's not a flaw in the WFCO - just a fact or life.