  We use the \textsc{Romulus25} cosmological simulation volume to identify the
largest-ever simulated sample of {\it field} ultra-diffuse galaxies (UDGs). At
$z=0$, we find that isolated UDGs have average star formation rates, colors,
and virial masses for their stellar masses and environment. UDGs have
moderately elevated HI masses, being 70\% (300\%) more HI-rich than typical
isolated dwarf galaxies at luminosities brighter (fainter) than
M$_\mathrm{B}$=-14. However, UDGs are consistent with the general isolated
dwarf galaxy population and make up $\sim$20\% of all field galaxies with
10$^7$<M$_\star$/M$_\odot$<10$^{9}$. The HI masses, effective radii, and
overall appearances of our UDGs are consistent with existing observations of
field UDGs, but we predict that many isolated UDGs have been missed by current
surveys. Despite their isolation at $z=0$, the UDGs in our sample are the
products of major mergers. Mergers are no more common in UDG than non-UDG
progenitors, but mergers that create UDGs tend to happen earlier - almost never
occurring after $z=1$, produce a temporary boost in spin, and cause star
formation to be redistributed to the outskirts of galaxies, resulting in lower
central star formation rates. The centers of the galaxies fade as their central
stellar populations age, but their global star formation rates are maintained
through bursts of star formation at larger radii, producing steeper negative
g-r color gradients. This formation channel is unique relative to other
proposals for UDG formation in isolated galaxies, demonstrating that UDGs can
potentially be formed through multiple mechanisms.

Astrophysics of Galaxies