How to interpret transitions in a QPSK constellation diagram?
Note how the second image looks kinda-sorta like two rotated copies of the top image superimposed.

What has happened is that the receiver has temporarily lost synchronization with the signal. In fact, it looks like it is off by exactly &plusmn;22.5&deg;, which suggests that there might be a problem with the phase detector specifically.

BTW, "spectrum analyzer" is not the correct term here &mdash; that would imply some sort of frequency-domain display. This is a time-domain display, with many cycles superimposed.