Don't Give Vitamin D as a single Annual Dose of 500 000 IU

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I am new at this, but I am missing the rationale why the authors of this study choose to give all of a year's supply of vitamin D on a single day?

This study compared giving 500 000 IU of vitamin D to placebo to 2256 community-dwelling women, aged 70 years or older who are considered to be at high risk of fractures. The  absolute risk of falls was higher in the vitamin D group by 10.7 falls per 100 people per year. And risk of fractures was higher in the vitamin D group by 1 fracture per 100 people per year.

The point that I am missing is the rationale for giving all the yearly supply of vitamin D together in one day. Do you know the rationale?

JAMA -- Annual High-Dose Oral Vitamin D and Falls and Fractures in Older Women: A Randomized Controlled Trial, May 12, 2010, Sanders et al. 303 (18): 1815