I also think the problem is caused by placing the floats to the nofloat cloumns block. Is there another way to this?
I find floats difficult. They do weird things. I tried to create a minimum file that shows your columns of figures behaving badly -- I didn't succeed. Here is the test file I created:
In your example, just make a small change you will see the failure. I attached the result pdf file. \setupcolors [state=start] \startuseMPgraphic{fig1} w := 4cm ; h := 2cm ; ww := 1cm ; hh := 1.5cm ; fill (0,0)--(0,h)--(w,h)--(w,0)--(ww,0)--(w,hh)--(ww,hh)--(ww,0)--cycle ; \stopuseMPgraphic \starttext \input tufte \input tufte \startcolumns[n=2] \placefigure[here]{blabla}{\useMPgraphic{fig1}} \column \placefigure[here]{blabla}{\useMPgraphic{fig1}} \stopcolumns \input tufte \startcolumns[n=2] \placefigure[here]{blabla}{\useMPgraphic{fig1}} \column \placefigure[here]{blabla}{\useMPgraphic{fig1}} \stopcolumns \input tufte \stoptext