J.M.
She may be holding it longer than is good for her when you go out. You know how if you need to go but don't you kinda loose the feeling that you need to go, even though you still do? MAybe once she reaches this point during the outing she's just not able to get that sensation and control back for a while. She's new at this and it takes time. I continued to put my son in a pull-up for outings long after he was potty-trained at home. He would never tell me he needed to go if we were out, he would just hold it until he got home and then dash for the potty. He still won't poop if we're out. I put the pull-up even though he didn't wet it "just in case" and I left it on for a while once we were home, at least until he pottied once.