Surely there are enough reasons to have a nice acidosis when you are full of ketones. But on top of all that, you can also get a lactic acidosis.
There is an association of diabetic ketoacidosis and lactic acidosis that goes beyond the tendency of diabetics to be on metformin. Indeed, in one case series there was “significant” lactate levels (i.e. over 4.0 mmol/L) in 40% of the patients. Glucose levels tended to correlated with lactate levels, suggesting that the lactate here is being produced as a result of some sort of abnormal cellular carbohydrate handing.
The authors of this recent paper have admitted that this association is poorly researched. There does not seem to be a good mechanism worked out at this stage. Again, endogenous catecholamine excess is implicated. It is known that adrenaline levels correlate with DKA severity.
D-lactate production may also play a role. A 2011 study has correlated D-lactate levels to the size of the anion gap in diabetic ketoacidosis; the article also ventures a hypothetical mechanism as to how this comes about.
In any case, a much earlier study has demonstrated that whereas the ketone body levels correlated well with pH, the lactate did not, which suggests that lactate plays a fairly minor role in the actual acidosis.
Additionally, Kerry Brandis reports on the interaction of lactate and ketoacids (specifically, on how lactate production tends to reduce the concentration of acetoacetate, thus fooling the most common ketone tests