To delete the data, if you mouse-over just under the little keyboard icon on the left hand side of the midi track, it will display an arrow, click on that, and it will show you the automation lanes. You can then either switch it on or off with the R and W buttons, or select/draw/delete the level of the parameter that you want
R turns on read automation but I doubt this is anything to do with your problem
Double click the event to open the midi editor, in the controller lane at bottom left click the little page icon next to the plus sign and choose show used controllers.
It will open up all the controller lanes and hopefully it will be obvious what is setting the volume…probably Main Vol cc7.