Eileen was kind of tilted, minding her own business and her body lazily slanted against the book shelf. Her eyes moved along the book pages and lines until she saw a pale hand twitch forward and a soft voice call out to make her move. Eileen wasn't someone that could just be careful and step to the side and be smooth. There had to be a dramatic distress for her.
As she moved her right leg back, the back of heel hit the inner part of the step, throwing off her balance. The book dropped from her hands and before she could balance herself out, she was thrown forward and her arms flailed. Eileen was going to make a fool out of herself and roll down the stairs like an idiot, getting possibly injured.
That is unless she grabbed something on the way down or Tom gripped her to balance her out.