On the assumption that sensei knows better than uke what they can "cope with"? Demonstrably, it was nage's judgment that was in error in this case...significantly so.
It ain't what you don't know that gets you...it's what you know that just ain't so.
All of these interactions have two sides or maybe many sides even given that nage or teachers responsibilities may lead....still when one is teaching you are looking at any number of inputs coming at you rather than the general one coming from teacher to students. At seminars you have even more diverse personalities, most you may have never come in contact with, looking up at you. Not all of these folks have the same agenda or are there for the same reason or have the same expectations........sometimes the "noise" these folks knowingly or unknowingly direct at you just gets to you as a teacher..... We are after all only human...having bad days tossed in sometimes making it easy to let go for a moment. Given we may all be trying to get to that point were we just turn and smile...resolving all conflicts and we should all try for that......
As for me I have only knowingly hurt two people in my 38 years of being involved in Aikido, both times as a result of a long build up of crap from the other person..... one I was sorry for afterwards and the other not so much. In both cases I could have left the mat giving up "my house" so to speak to someone who knew what they were doing....doing it for that very reason. In taking the 10,000 falls I have taken over the years I can recall only about 10 people who were seriously trying to do damage to me on the mat.... I think that is not bad as a general comment.
I don't favor what happened in the mentioned seminar though it is clear through my experiences there are a number of reasons it may happen and sometimes in the "moment" we can't control everything.