If I sound nerdy I apologize, I love ME2.
Everyone in ME2 has a score, ranking how strong they are and how likely they are to live. Mordin has the lowest score since he's a salarian scientist and probably someone like Grunt has the highest score. The score isn't determined by loyalty, loyalty determines focus, and focus can get anyone on your team killed if they dont have it. If you send a strong individual on a mission, who is loyal and is right for the job then hey thats great. But what people need to keep in mind is that group firefights also have a set score and if its not met then someone with a low score dies. If, on the final decision, you send Grunt with the survivors. Take people like Garrus and Jack with you, and send someone lame to lead the fire team, there will be alot of dead people since the fire teams score will be very low since there arn't alot of strong people to help them. Tali died in your game because her score is low since shes a sickly Quarian. Jack died because she lacked focus
SO, my answer to you, Mordin died randomly in the fire team fight because I sent all the strong people I had away to do other things or fight with me. This is why Miranda and Thane are on the front cover of the ME2 box art. Taking them insures no deaths in the fire team since their scores are average, and they wont die with you during the fall if they're loyal to you. Miranda died in my first play-through due to me picking Jack and not being a super renegade/paragon.