If all HBD exceeds 10% of the value of all HIVE then HBD will lose some of its backing and could (probably would) drop below $1 in value until the amount of HBD was reduced or the value of HIVE went up.

Right now it is closer to 2%, so not a concern short term, but if the value of HIVE were to drop a lot and/or the amount of HBD being held were to increase a lot (without HIVE going up in value), then it could be.

There is a plan to increase the limit in an upcoming hard fork (to I think 20% or 30%, I'm not sure which), so the limit may become even less likely to be hit, but that change is not certain to happen.