I believe this is due to some discontinuity in the dechirped signal. For example, when the signal is at first range bin, then the first sample in the dechirped signal is 0. If in your program, you do
before passing it through fft, the phase angle becomes 0. Similar result is obtained if for the signal arriving in second range bin, I do
before taking the FFT.
This being said, is there any particular reason you are looking at the phase angle? The range information is recovered from frequency so I'm wondering what you plan to use the phase angle for. Thanks.