trying to display unusual matrix reboots calculator

[webmasterpdx]

I set a matrix, m to [[1 2 3],[4 5 6],[7 8 9]]
Then I was trying to see how the at() undocumented function works. So far, I'd only used it on vectors, so I wanted to try setting position in matrix to 4, thinking it might be the first element on second row, but instead it created a 4th row equal to one element (no longer a matrix).
i.e. m[4]=<99;
So this just added a 4th column/row and set it to 99, so it's no longer a square matrix.
When I put it in matrix mode and clicked on it, it rebooted the calculator.
Why did it do that?