Hi R.,
It is very common for children who have "mastered" toilet learning to regress. There are typically many triggers for this, but change and stress are usually big ones that come into play. The change could be as sudden as something new that might have happened last week or something that she had 'adjusted to' a month or so ago. Changes in living arrangement (if your spouse has been deployed or returned recently), care arrangements (where and with whom she spends her days), or if she has moved homes recently can all be big factors. Even good changes can be stressful, and kids don't usually have the tools or self-knowledge to articulate their feelings; consequently, they manifest themselves in regressive behavior. Think of it as a short-circuit of sorts, a cue that "something's not comfortable" in their world. One child I know even began regressing before a big change (starting a new school) because her parents were so busy talking about it and preparing her for it months in advance that she simply felt overwhelmed, although the actual change hadn't yet occured.
Another cause of wetting could be a child's knowledge of their selves, and wanting control of their own lives, to a degree. I've worked with kids who asked repeatedly for diapers, and when denied, have had accidents. The children knew, at that time, they needed a little extra help. Perhaps they needed a break from constantly having to be tuned into their bodies cues, to stop what they enjoyed doing and use the bathroom. Personally, when kids ask for diapers, I have learned that it is least damaging to our relationship and their self-esteem try to accomodate them. This also diminishes the chance of it becoming a power struggle.
There's one other option, and that is that the regression is being continued by a desire for attention. Kids who have accidents get a lot of attention, even if it is the form of an upset adult. If children are acting out of a desire for attention, it won't matter to them that it is a positive or negative interaction-- attention is still attention. Your daughter is at an age that simply taking her to the bathroom and handing her a plastic bag to put her wet clothes in is all that's necessary in way of interaction. If it were me, I wouldn't even venture any emotional comments, but would keep the conversation straightforward. "Oh, you've had another accident. Well, come to the bathroom, you'll need to change yourself." I'd leave some wipes in there too, and just do the bare minimum to keep it from becoming a mess.
In cases like these, I like to keep conversation about it to a minimum. Some adults rail on and on about the accidents and messes, "You're too old for this" or "I thought you were a big kid!" and assign some sort of consequence to it. Others go the opposite way and begin to offer sticker charts, rewards and 'incentives'. I think there's a danger in punishing or rewarding someone's struggle in an area they have already mastered.
Refrain from commenting on it in front of your daughter to other people. She is obviously struggling with something, and the last thing that will help is to gain a sense that messes are becoming "expected" of them.
For what it's worth, accidents and pants messing is rarely an outward aggressive behavior, although we tend to treat it as such. Usually, it's a genuine struggle within a child, coping with stress, change, or figuring out how to regulate themselves while having the freedom they desire. In short, treating it like "bad behavior" is typically counterproductive to it not happening again.
I know this is a lot to think about, but I hope there is something in here that rings true to your situation. All of the kids I've worked with that experienced regression were great kids. They weren't using these accidents to act out, however, how the families each handled these situations really determined how quickly each of them were resolved, and to what degree the child's self-esteem was affected. I know it's hard, but try to be cool, ask your daughter what she thinks she needs, and eventually, this too will pass.
