"this happens mostly" is correct and usable in written English.
You can use it when you want to indicate that something occurs more often than anything else. For example: "This happens mostly during summer months."
This happens mostly because the DICOM standard does not comply with distributed fetching of image fragments while performing a store.
There's a reason I cannot seem to get my blog finished in breathable time -- inevitably I back myself into an adrenalized writing frenzy to finish before deadline -- but this happens mostly during the summer.
This happens mostly because information about their location is hardly to retrieve in a well arranged form.
This happens mostly when in a less crowded world.
This happens mostly after you fill out a lead form on a website which contains your phone number and email.
It's important to note that we're seeing this happen mostly among hedge funds that focus on tech.
This happened mostly because the company offered a compelling product: a simple, flexible-document database that many developers found much easier to use than older relational databases.
In a small number of cases, the chirality information is missing in the SMILES of the current study, this happening mostly for racemic entries for which the compound name in the CIF specifies a single enantiomer and also for compounds containing sulfoxide groups, since OPSIN sometimes generates chirality information for them, this not complying with the SMILES specification.
All this happened mostly because it was just kind of your turn, and you take it in stride.
We stormed the headquarters of the foot-dragging Food and Drug Administration, and a few windows even broke in the process (no one was hurt and this happened mostly from overzealous police).
This happened mostly for microsatellite dsub14 due to a temporal increase in heterozygosity in this particular locus [see Additional file 1].
