Girl likes going limp for fun?

A girl I know very well likes me having my arm around her and being held, but sometimes she goes ... strange. Often when we're standing, and I'm hugging/holding her, she lets herself go all limp and floppy so that I have to use all my strength to hold her up. Shes not very heavy but sometimes its a delicate balancing act (literally).I don't really mind so much because I like hugging her lots but I don't get why. Is it just because its fun and she really trusts me, or does she do it on purpose so that I hold her tighter? I'm a guy btw. She does this very often, nearly every time we meet she will in some way make me physically support her in this way. Our relationship is best defined as one of closeness, not family, not intimate, just, really really close. Have no idea if she has a thing for me or whether she thinks of it any deeper than just light fun. I'm just wanting to know whats going through her mind when she acts like thins, any of the female persuasion wanna chime in on this? Or guys, ever had a girl act like this around you? If so was this more the behavior of a gf, sibling, or really close friend?