A change in fault dip is often a sign that the fault has reactivated due to a change in the stress state or may be due to the impact of different lithologies (shear strength).
Note how the fault changes angle from almost vertical in the pure diatomite (Fig.1) to a gentler dip in the overlying more clay rich, bioturbated sediments. The 2nd example (Fig. 2) is from a limestone - shale sequence at Kilve Pill Somerset, UK. Note how the fault steepens through the limestone beds (yellow/brown) but is more gentle through the shales (grey).
Figure 1. Fault changes angle from purest diatomite in the sequence (shallow marine) into more clay rich, bioturbated sediments. Carpenter Canyon, California, US. Used by kind permission of Jon Schwalbach.
Figure 2. Fault refraction through limestones and shales, Kilve Pill, Somerset, UK.
A seismic example of how faults change angle is given in Fig. 3. The 2 most through-going faults (labelled 1 & 2) have significant displacement at basement level (100s of metres). This decreases upwards and fault 2 changes dip at approximately 1200 m TVDSS, which coincides with the Tertiary/Cretaceous boundary. This may be due to a reactivation of the fault (under a different stress field orientation) or simply that there is a change in rock properties from tight Cretaceous muds to porous Tertiary sandstones, the steeper section of the fault resulting from propagation through material with a higher friction angle.
Figure 3. Example of fault refraction on a seismic section from Clair Field, West of Shetland (Ogilvie et al. 2015). It occurs at the Cret-Tert boundary which suggests that a lithology change from dominantly shales to sandstones is the driver. A change in stress state is also possible.