Sudden accelerometer value changes, and quad become rocket

Here is the screenshot of log.
I was flying althold mode, and during the flight, I’ve changed to pos_control mode.
Then, suddenly, accelerometer reading is 60m/s^2 as shown in the graph.

What happened? Quad just become rocket, and max throttle was logged (see below)

Anyone knows what the problem is??

Probably better if you attach the complete log and a little description of the vehicle.