Threaded push! different for supposedly similar array initializations

[jling]

julia> Y = fill(Int[],3);

julia> push!(Y[1], 9)
1-element Vector{Int64}:
 9

julia> Y
3-element Vector{Vector{Int64}}:
 [9]
 [9]
 [9]


julia> X = [Int[] for i=1:3]

julia> push!(X[1], 9)
1-element Vector{Int64}:
 9

julia> X
3-element Vector{Vector{Int64}}:
 [9]
 []
 []