Both Secret Service agents had hands on Fields, so they must have been concerned about something. If they didn't actually touch her, they were about to.
When the Secret Service first moves on Fields, Lewandowski was looking away and not yet aware of the situation.
For those who question why the Secret Service let her go in the next still shot, they are not police. Their job is to secure Trump. Trump was walking on, so after they reached for Fields and then she stopped because Lewandowski reached for her, they immediately moved back to surrounding Trump. They would not have detained her if it meant having Trump move beyond their reach. Fields was no longer threatening Trump's protected zone because Lewandowski stopped her.
Also notice that Lewandowski did not yank her back, he just stopped her forward progress. When Trump moved on, so did the Secret Service, followed by Lewandowski.
-PJ