  We examine sources of scatter in scaling relations between galaxy cluster
mass and thermal Sunyaev-Zeldovich (SZ) effect using cluster samples extracted
from cosmological hydrodynamical simulations. Overall, the scatter of the
mass-SZ scaling relation is well correlated with the scatter in the mass-halo
concentration relation with more concentrated halos having stronger integrated
SZ signals at fixed mass. Additional sources of intrinsic scatter are
projection effects from correlated structures, which cause the distribution of
scatter to deviate from log-normality and skew it towards higher inferred
masses, and the dynamical state of clusters. We study the evolution of merging
clusters based on simulations of 39 clusters and their cosmological environment
with high time resolution. This sample enables us to study for the first time
the detailed evolution of merging clusters around the scaling relation for a
cosmologically representative distribution of merger parameters. Major mergers
cause an asymmetric scatter such that the inferred mass of merging systems is
biased low. We find mergers to be the dominant source of bias towards low
inferred masses: over 50% of outliers on this side of the scaling relation
underwent a major merger within the last Gigayear. As the fraction of
dynamically disturbed clusters increases with redshift, our analysis indicates
that mergers cause a redshift-dependent bias in scaling relations. Furthermore,
we find the SZ morphology of massive clusters to be well correlated with the
clusters' dynamical state, suggesting that morphology may be used to constrain
merger fractions and identify merger-induced outliers of the scaling relation.
