fill will re-use the same element multiple times, not make independent copies. Thus:
julia> tri = fill([1], 3) # the same array, 3 times
3-element Array{Array{Int64,1},1}:
[1]
[1]
[1]
julia> push!(tri[1], 2) # mutate the sole inner array
2-element Array{Int64,1}:
1
2
julia> tri
3-element Array{Array{Int64,1},1}:
[1, 2]
[1, 2]
[1, 2]
julia> tri[3] = [3,4,5]; # replace an element of the outer array
julia> tri
3-element Array{Array{Int64,1},1}:
[1, 2]
[1, 2]
[3, 4, 5]