Close all tabs except rabb.it and hold that at one tab if possible.
See if you get "closed out". If not, open another tab, and watch more, Repeat keeping track of the tabs.
Could be that some number of tabs reaches a threshold value and your browser, rabb.it, or some system process or service cannot handle the load and causes the reset.
Try some controlled experiments to see if you can do things (browse, open tabs, etc.) and cause the reset.
Then you can use Task Manager and Resource Monitor to watch what you system is doing while you intentionally set up a reset.
Likely to be some resource being overloaded or a conflict that occurs as a result of multiple tabs.
What OS and browser are you using?