The phrase "a usual reason" is correct and usable in written English.
It can be used when referring to a common or typical justification for something.
Example: "The usual reason for delays in shipping is a backlog in processing orders."
Alternatives: "a common reason" or "a typical reason".
Dental fear was found to be related to less frequent dental visiting, whether measured by past behaviour or future intentions, more prevalent dental problems, whether assessed by the number of teeth missing, perceived need, social impact or self-rated oral health, and symptom-driven treatment as measured by a person's usual reason for visiting.
When a museum gets headlines for anything but a new building or a fine exhibition, the usual reason is bad news.
In addition, 67.3% of people with very high dental fear reported that their usual reason for a dental visit was for a problem compared with only 44.9% of people with no dental fear, 47.1% with a little fear and 45.8% who were quite afraid (χ2 = 121.03, p < 0.001).
At school, the only subject I was any good at was music, and for the usual reason: an inspiring teacher.
The UK, of course, doesn't have its own national day - partly because we are collection of countries and partly, according to some, because when others were setting their ones in the calendars, we were too busy taking and then granting independence (the usual reason for a "day) to other countries to get time to create one.
The usual reason for judging a trial's quality as inadequate was weakness of blinding.
Lois seems a bit perplexed by this, given the usual reason most men use Grindr.
Alonzo went into journalism for the usual reason: "I always wanted to be a writer".
The usual reason for withdrawal is determination of a previously unknown yet highly undesired side effect.
