That could be malfunctioning hardware, turned off AIS, or a gap in recording. Smaller deviations from a line are most likely GPS jitter. AIS is transmitted over VHF. Terrestrial stations listen for the transmitted AIS messages to record them for public consumption.
Source: I collect AIS data over TCP/IP directly from my orgs ships.
Source: I collect AIS data over TCP/IP directly from my orgs ships.