Tectonic activity can change the volume of oceanic basins and hence, modify the sea level, ice not involved.
Firstly let's explain the left picture.
At Cretaceous the tectonic activity was high. The flux of material from mantle to ocean ridges and plateaux reduced the volume of ocean basins, wich produced a global high sea-level observed in differents parts of the globe.
During Campanian and Mastrichtian global transgressions are recognized in several stratigraphic sections of the Earth.
Z. Dubicka and D. Peryt (2012) studied a stratigraphic sequence in Poland wich also showed transgression phases on Mastrichtian. They correlated them with other strata of the age.
This sea-level rise, recorded in south-eastern Poland, can be
correlated with the transgressive peak recognized by Hancock (1993);
this corresponds to the Craise Graise (¼ Vijlen Member) exposed in the
CPL quarry at Halembaye in Belgium, which in turn corresponds to the
lower Belemnitella junior Zone, i.e., earliest Late Maastrichtian.
Hancock (1993) suggested that this peak may occur globally. He
correlated it with the Prairie Bluff Chalk on the northern margin of
the Gulf of Mexico and with strata that lie a little above the base of
the Tinton Greensand on the Atlantic coastal plain. The transgressive
peak in the earliest Late Maastrichtian was also documented in the
Sopelana section in the Basque Arc Domain in Spain (Gómez-Alday et
al., 2004), which coincides with the inoceramid extinction, the
informal definition of the Lower/Upper Maastrichtian boundary.
Moreover, a marked transgression in the middle Maastrichtian was
reported from the Southern Hemisphere by Huber and Watkins (1992) who
noted a higher diversity of keeled foraminifera, which they
interpreted as reflecting a possible transgression.
Then let's focus in your question about the sea level drop. It is pretty obvious a regression happened, at least at North America.
The sea level drop appears to be global too, according to Matsumoto (2002) and associated to a slowing down of global tectonic activity.
Tectono-eustasy may have been the main cause of the phenomena of
transgression-regression, but certain kinds of other tectonic
movements which affected even the so-called stable platforms were also
responsible for the phenomena. The combined effects of various causes
may have been unusual in the Cretaceous, since it was a period of
global tectonic activity. The slowing down of this activity followed
by readjustments may have been the cause of the global regression at
the end of the Cretaceous.
Zofia Dubicka, Danuta Peryt (2012): "Latest Campanian and Maastrichtian palaeoenvironmental changes: Implications from an epicontinental sea (SE Poland and western Ukraine)", Cretaceous Research
Volume 37, October 2012, Pages 272-284.
T.Matsumoto (2002): "Inter-regional correlation of transgressions and regressions in the Cretaceous period", Palaeogeography, Palaeoclimatology, Palaeoecology. Volume 187, Issues 1–2, 1 November 2002, Pages 35-60