I am not familiar with any mechanism that would allow dextran conjugates to spread over chemical synapses. Researchers have put a lot of effort into developing tools to allow such transsynaptic tracing. 

Depending on the size of the dextran and which neurons you are filling, a more likely explanation is that you are seeing gap junction coupling. Especially interneurons tend to show strong gap junction (= electrical synapse) connectivity and it is common to see fluorescent dyes slowly labeling neighboring connected interneurons. 

Both, gap junction and bulk endocytosis are possible. For the enodcytosis, you need however to assume that the dextran is first released from the presynaptic terminal, which is highly unlikely.