Broadly speaking, engaging in a task activates the task-positive network, which actively suppresses the default mode network [1][2][3]. This likely happens during meditation [4], as during properly done meditation, one is supposed to be focused on the task of breathing. But this can be interrupted, particularly by stray thoughts/salience/attention shifts [5].

[1] "The human brain is intrinsically organized into dynamic, anticorrelated functional networks", https://www.pnas.org/doi/abs/10.1073/pnas.0504136102

[2] "The Brain's Default Mode Network", https://www.annualreviews.org/doi/10.1146/annurev-neuro-0710... {search for 'task-positive'}

[3] "Neural antagonistic mechanism between default-mode and task-positive networks", https://www.sciencedirect.com/science/article/pii/S092523122...

[4] "Mantra Meditation Suppression of Default Mode Beyond an Active Task: a Pilot Study", https://link.springer.com/article/10.1007/s41465-017-0028-1

[5] "Modulatory interactions between the default mode network and task positive networks in resting-state", https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4017816/