There seems to be a problem with the structure information added when using
\startmatrix:

\setuptagging[state=start]
\starttext
\math{
  \startmatrix
    \NC A \NR
    \NC B \NR
  \stopmatrix}
\stoptext

The log reports an open chain: document>1 => mtr>1 => mtd>1 => mtr>2 =>
mtd>2 => mtr>3
This may be a bug.