Like everywhere else, there is a lot of competition on the motherboard industry, as companies race against the others in their quest to secure contracts and enchant customers.
ASUS has led in terms of shipments for quite some time, but Gigabyte is a close second. A very close second if recent reports are to be believed.
During the first quarter, Gigabyte shipped 4.9 million motherboards, just a little behind ASUS' 5 million.
Thus, by the end of the year (2013), ASUS may very well be relegated to second place.
Since the two accounted for 47% of the total motherboard market in 2012 (40 million), the change in ranking won't really affect consumers much, although the reputation of being “the best” could color opinions here and there.
That said, ASRock will probably keep the third spot, unless MSI somehow supplants it (in 2012, they shipped 7 and 6 million, respectively).