what is the reason for the Change in DFT analysis after downsampling firstly the original signal's dft analysis is this the signal is sammpled at 1000Hz


the signal is then downsampled under different rates such as 25Hz

this is the dft analysis of the signal downsampled under 25Hz


what is the reason for this phenomenon?
Because you have resampled the signal to a sample rate below twice the highest frequency in the signal, you are seeing the signal plus an aliased image of the signal both summed together.  Two partially overlapped rectangles will sum to produce the "head-and-shoulders" shape you see in the aliased spectrum.