It comes from the css definition in style.css around line 125. There is this rule:
.hotel-detail-booknow a, a:active
defined that gives the link its enormous size and blue colour. I would guess that it should be:
.hotel-detail-booknow a, .hotel-detail-booknow a:active
In your case, the rule is applied to all clicked links, not just the ones within a .hotel-detail-booknow element.