I don't have a physics background. (I'm a Computer Science student)

I have a waist-worn accelerometer provided by the school which is quite reliable. I've captured data of sitting still to standing still transitions from a person. Here is the result synchronised from many tests, the orange curve depicts data from the vertical axis. 
[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/yfuxD.png


Could anybody please explain to me the pattern of acceleration data?
Why does the acceleration's magnitude always increase first before decreasing? (The sign of value depends on the device's configuration so you don't need to worry about it)
Should it not be the other way around? 

As far as I know about physics, Newton's 2nd Law states $F = m . a$ so I would think that when the person stands up, a force in the upwards direction is created which is opposite the the Earth gravitational force which heads down so the resulting force must be lower than the original force (which only includes the Earth gravitational force) leading to a reduction in the acceleration?

Also if possible could you let me know what possibly causes the sudden drop in acceleration right after that? 

Many thanks.