She doesn't want to get it wrong. Because your preschooler can do more than ever, she's afraid she'll lose face if she makes a mistake. Praising her excessively for new achievements can pressure her to nail them every time. If you sense she's nervous, resist the urge to come to her rescue. By stepping back, you show your confidence in her. If she doesn't budge, help her, but give her a chance to try again on her own later on.
She's in an unfamiliar situation. Activities she's mastered seem much tougher in different contexts. Having to spell her name for a new teacher, or buckling her seat belt on her first plane ride, can be scary for her. If you can wait, give her time to get used to the new setting, then prompt her to try once more. Keep her focused on the task at hand, instead of what's going on around her.
She's afraid you'll stop taking care of her. If she can do some things herself, she may think you won't do things she can't do herself, like tucking her in at night or cooking her dinner. Plus, every big kid wants to be babied sometimes -- especially if there's a baby in the house. Remind her that you're still there to help, snuggle her up, and put her socks on for her this time. But tell her that next time, you'll watch her put them on.