She could be testing her limits & control. My daughter went through a period where she'd get really involved with playing and wait till the absolute last second before deciding to go pee, then she'd run (or shout pee-pee) for the potty and make it there, but have damp undies.
UTI is another possibility--if there's any indication of one, it can't hurt to get a culture done.
Constipation is another possibility. It's much harder to control your bladder when you're constipated.
Changes in hydration can be difficult for newly potty trained kids to adjust to (ie heat waves, or cold spells where their liquid intake changes)
Illness (or teething) can cause less bladder control in young kids. Even before they get sick sometimes poor bladder control can indicate an illness coming on, because the kids are struggling to fight off the sickness, and feeling lousy, so they don't notice their bladder getting full.
Food/environmental reaction is another possibility. If your daughter is showing any signs of a reaction (sudden mood changes, flushed skin, eye rings, reddened vulva/anus, rash, etc) this could be the cause for her lack of bladder control.
Talk to her about it. Explain sometimes we have to be proactive about using the toilet, so our "play time" isn't interrupted. Find out if she is surprised about the accidents (possible reaction to food/environment) or if she was just putting off using the toilet. Ask her if she's having any pain when she pees (indicator for UTI or other urinary problems). Check on her bowel movements, less than once a day, consider she may be a bit constipated.