Question about differential amplifier latchup

[Bitrex]

There are some great videos on YouTube about Analog IC design from the Indian Institute of Technology that have a lot of material that is very applicable to audio design as well. In one of the videos on differential amplifier characteristics the professor mentions that early generations of op amps had a problem with latchup, where when the common mode voltage to a differential amplifier with a current source tail and resistive load became excessive the negative feedback around the amplifier would turn positive and cause the output to slam to one of the rails and stay there. He also states that using current source loads helps ameliorate this problem.

I'm having some trouble first understanding why the change from negative feedback to positive feedback occurs - is it because when the input voltage to the differential pair rises above the collector voltage the transistor has its base to collector diode forward biased and is behaving kind of like an emitter follower? Also, how does using a current source load help solve this problem? Any help in clarifying this issue in my mind would be greatly appreciated.