My first guess it's the autopilot disengaging the VNAV because of the overspeed. Remember the aircraft is trying to slow down from 290kts to 240kts when passing 10.000ft If you're 15knots above the VNAV target speed speed (in your case 240 kts below 10.000ft) it will revert to another mode
For example: (LVL CHG + MCP SPEED + LNAV)

Solution: use the inflight speedbrakes, just for maybe 15 seconds. Maybe a tad uncomfortable for the pax, but You'll decrease the speed  

Edit: I think the 15kts overspeed limit only applies when you're below 10.000ft, not really sure.