I think Andi was mad at Eric for poking a hole in one of the central premises of the show, which is that the lead is actually seriously considering as a romantic partner all the suitors left on the show. There can't be more than 3 or 4 guys left at this point that Andi is really into, but she can't ditch all the other guys at once, because that would make for a really lame (or really short) season.
So she has to keep stringing all the other guys along, making them think she's interested, because otherwise they might figure out that she's not that into them and ditch the show. Also, if we believe that the lead is SUPER SERIOUS about all the remaining guys, then it makes it more okay that she's encouraging them to pour their hearts out and 'put themselves out there' and whatnot. Except that Eric realized that this was what Andi was doing and called her bluff - he realized that even though she asked him to open up, she wasn't doing the same thing, because he was one of the B-roll guys (I'm pretty sure that 'our relationship is moving more slowly' is bachelor speak for 'I'm just not that into you.') So he tried to talk to her about it - basically, he said, hey, I feel like I'm being real and vulnerable and I'm not getting that from you - and what Andi heard was "I see through the facade, and you're a bad bachelorette." And then the explosion, where he said she was acting and not being real, and she started going on and about HOW HARD she was working. That... doesn't make any sense. Except in Bachelor world.