Support for overflow: hidden on the <body> element is quite limited in iOS and Android. To that end, when you scroll past the top or bottom of a modal in either of those devices' browsers, the <body> content will begin to scroll. See Chrome bug #175502 (fixed in Chrome v40) and WebKit bug #153852.
As of iOS 9.2, while a modal is open, if the initial touch of a scroll gesture is within the boundary of a textual <input> or a <textarea>, the <body> content underneath the modal will be scrolled instead of the modal itself. See WebKit bug #153856.
As of iOS 9.3, while a modal is open, if the initial touch of a scroll gesture is within the boundary of a textual <input> or a <textarea>, the <body> content underneath the modal will be scrolled instead of the modal itself. See WebKit bug #153856.
Also, note that if you're using a fixed navbar or using inputs within a modal, iOS has a rendering bug that doesn't update the position of fixed elements when the virtual keyboard is triggered. A few workarounds for this include transforming your elements to position: absolute or invoking a timer on focus to try to correct the positioning manually. This is not handled by Bootstrap, so it is up to you to decide which solution is best for your application.