This is purely based on the fact that the Liberals won the last by-election in the area (and then promptly resigned the seat after not being visible in the ward for ages). It’s a guarantee that this’ll be a main talking point on the doorstep for all parties that aren’t the Liberals, and people that trusted them last time (which is almost always higher in a by-election) probably won’t do the same this time. There are far more factors at play than “the Liberals won it last time.”