Maybe they weren't having safe s*x, did it without a condom, a condom broke, they got the wrong condom size, maybe they were on birth control but missed a day and that was the day it happened, ect.
Also, no. Adoption is not an alternative to abortion. Adoption would be the alternative to keeping the kid they can't take care of.
And theres more reasons to get an abortion. For example, giving birth could have health risks for the person, or maybe their body can't handle the pregnancy. There are medical reasons, not just situational ones.