Note, as said above, that fill will initialize the array of arrays with the same array:
julia> a = fill(Int[],2,2)
2×2 Array{Array{Int64,1},2}:
Int64[] Int64[]
Int64[] Int64[]
julia> push!(a[1,1],1)
1-element Array{Int64,1}:
1
julia> a
2×2 Array{Array{Int64,1},2}:
[1] [1]
[1] [1]