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