This is a discussion on BB Vs SB shoving range within the online poker forums, in the Tournament Poker section; How do you determine the range of your SB?
Lots of time you only been 1-3 orbits with them and this is the first time
They were probably stealing, but you are a coin flip to almost any two cards. You can coast into the money, but once you get there you will be dangerously low in chips. So it probably is decided on how much you want 2X your entry.
53-55% equity is definitely enough to call here with 15 BBs, regardless if you're near the money bubble or not.
If they think you're trying to get into the money or think you're folding a high number of hands and only calling with the top of your range, they can effectively shove much wider, giving 66 more equity to call.