It's most probably part of your dimension style setup.
Type DIMSTYLE, select the style the dimension you want to change is, and click modify.
On the LINES tab (top left) there's a box on the right "Extend beyond dim lines," have a play with this value.
On the FIT table (fourth in from the left) are a series of radio button under "fit options" which control how AutoCAD behaves if there isn't enough space to put both the arrows and text between the extension lines. Run through these options until you start getting the behaviour you want.
Depending on the scale of the preview you may need to OK out of the dimstyle editor and update your dimensions.
HTH
dJE
