The manual warns that "a safe speed on a curve for a straight truck or single-trailer combination vehicle may be too fast for a set of doubles or triples." What is the reason for this?

49 CFR 383.115(c); AAMVA CDL Manual Section 7.1 "Pulling Double/Triple Trailers" introductory paragraph ("less stable than other commercial vehicles"), with the curve-speed warning itself in subsection 7.1.1 "Prevent Trailer from Rolling Over," both p. 7-

This explains why a driver can't just reuse their "safe curve speed" instincts from driving a straight truck or single trailer — doubles and triples need their own, more conservative judgment on every curve and ramp.