Why does arithmetic left shift of negative number leads to positive number?
$01100 \Rightarrow +12$, not $+24$.

And it is $+12$ because $32-20=12$. With $5$ bits and $2$s complement you can only represent numbers in the range $-16$ to $+15$. You have overflowed this range and, in effect, "run around" to the other end and into the positive number range.