What is the cause of the increase voltage at the end of the transmission line during low load?

it is the Ferranti Effect. Lightly loaded long transmission lines behave like capacitors in parallel and thus generate reactive power which increases the voltage at the receiving end of the line. The lumped parameter approach to medium length lines may not be sufficient or adequate to account for this effect since the pie equivalent model lumps the shunt admittances together and divides by two and applied half at each end of the line model. A distributed parameter approach is more accurate where you account for the capacitive effect of the long line and factor that into your analysis as you integrate your model equation over the total line length. One way of minimizing this Ferranti effect is to implement transposition into your line design, to cancel out or minimize this capacitive effect of lines: abc, cab, bca.