$.$Based on the following Life$-$Cycle S$-$Curve and internal conflict level, three possible separate$[1]$ reasons can contribute to the sudden increase in the slope of the actual progress curve from point A to point B:

$$ i$.$ The Project Manager$$s early performance was poor, the Senior Management intervened at point A$,$ resolved the problems, and gave the project back to the Project Manager to continue running it$,$ when the crisis was over.

$$ ii$.$ The Project Manager was not empowered $($supported$)$ properly by the Senior Manager.$$ She was given the right people, but not the right tools & equipment.$$ At point A$,$ she managed to convince the Senior Management to provide some necessary resources and make some ad$-$hoc improvements.

$$ iii.$$ The Client has been failing to finance the project properly and finally injected some money into the project at point $($A$),$ when he finally felt the danger.

$[1]$ In real life, projects fail due to a combination of these $($and other$)$ reasons.$$ For this exercise, for simplicity, we are assuming only one party is at fault $($PM$,$ SM$,$ or the client$),$ not a combination of them.