Adjacent-track interference (ATI) in a dual-layer perpendicular recording system arises during both writing and reading. During writing, the stray fields from the write head can cause side writing and side erasure. A dc-erase method using media with a low nucleation field is used to identify the extent and polarity of these fields. In media with a strong anisotropy in the soft underlayer (SUL), experimental results reveal that the fields causing ATI have a strong cross-track asymmetry due to interactions between the writing flux and the magnetization of the SUL. On readback, cross-track amplitude profiles taken at long wavelengths show a distinctive "side bump" predominantly on just one side of the track profile. This asymmetry is again found to be associated with the magnetization in the SUL. A theoretical study yields expressions for the lateral extent of the side reading and a critical bit length at which side reading becomes significant. Both of these are consistent with experimental observations. The onset of side erasure during writing is further equated with the time-dependent nucleation field of the medium. Theoretical arguments on thermal decay in the presence of external and demagnetizing field indicate that the threshold for ATI-associated side erasure is expected to follow a time dependence similar to Sharrock's formula for dynamic coercivity.