I'd say its a bit unstable because the ERC has been going on for so long. The 5pm discussion explains it somewhat though:

HURRICANE RITA DISCUSSION NUMBER 21
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 PM EDT THU SEP 22 2005

RITA IS GOING TROUGH THE WEAKENING PHASE OF AN EYEWALL REPLACEMENT
CYCLE AND DATA FROM THE NOAA RECONNAISSANCE PLANE INDICATE THAT
WINDS HAVE DECREASED TO 125 KNOTS...AND THIS MAY BE GENEROUS.
HOWEVER...THE MINIMUM CENTRAL PRESSURE HAS REMAINED AROUND 913
MB...WHICH IS A VERY LOW PRESSURE TO HAVE ONLY 125 KNOTS. IN
ADDITION TO THE EYEWALL REPLACEMENT CYCLE...RITA IS CURRENTLY
MOVING OVER THE EDGE OF A COLD SST EDDY. THIS COULD HAVE ENHANCED
THE WEAKENING TODAY. THE HURRICANE IS FORECAST TO MOVE OVER ANOTHER
WARM EDDY DURING THE NEXT 12 TO 24 HOURS AND THERE IS SOME CHANCE
THAT RITA COULD REGAIN SOME INTENSITY. BECAUSE THE SHEAR IS
FORECAST TO INCREASE...THIS MAY COMPENSATE FOR THE STRENGTHENING
THAT MAY BE CAUSED BY THE EFFECTS OF THE HIGH HEAT CONTENT. THE
BEST OPTION AT THIS TIME IS TO KEEP RITA AS A 125 KT HURRICANE WITH
A SLIGHT WEAKENING BEFORE LANDFALL. THE BOTTOM LINE IS THAT RITA
IS EXPECTED TO MAKE LANDFALL AS A DANGEROUS HURRICANE OF AT LEAST A
CATEGORY THREE INTENSITY.

RITA IS MOVING TOWARD THE WEST-NORTHWEST OR 300 DEGREES AT 8 KNOTS.
THE FORECAST TRACK HAS NOT CHANGED AND RITA IS EXPECTED TO BE
STEERED TOWARD THE WEST-NORTHWEST AND NORTHWEST TOWARD THE
UPPER-TEXAS OR THE WESTERN LOUISIANA COASTS AS THE HIGH SHIFTS
EASTWARD. AFTER LANDFALL...STEERING CURRENTS ARE EXPECTED TO WEAKEN
AND THE CYCLONE COULD MEANDER FOR A COUPLE OF DAYS IN THE VICINITY
OF NORTHEASTERN TEXAS...PRODUCING HEAVY RAINS.