So, there's been no official studies on this phenomenon, but I'm fairly sure that there's little filing cabinet system in your brain with all the information you've collected over the years. And if you start to overfill the drawers, especially the one labeled "Languages," the little file clerk in your head says, "Oh dear. I suppose we must just start throwing things out then!"
I formed this most excellent theory based on my own very solid and extensive evidence: the English has started to tumble out of my ears.
I've caught myself framing English sentences as they would be in Spanish: "How do you call that movie?" instead of "What is that movie called?"
I use some phrases in English more now because they're direct translations of phrases I use in Spanish. Like "more or less" as a translation of "mas o menos" instead of using "about" like I used to say. "I think it will cost 10 dollars more or less."
I occasionally forget words in English. "And then they have to pay a...umm...a multa. You know, like, they charge you for doing something bad." "Fine?" "Yes, that!"
I sometimes stick Spanish filler words or conjunctions in English conversation. "Bueno, I need to do homework now." Or "Oh, yes, but the professor was late tambien."
Of course, I still have the same problems in Spanish.
I've noticed that we as study abroad students often like to "complain" about these slip-ups amongst ourselves or to other people. But of course the secret, which is not much of a secret at all, is that we're really quite proud of them. Hopefully they mean we've internalized the language to a certain extent.
If the little file clerk is already throwing things out, it must mean we've shoved in quite a lot of Spanish, right?
Or we just had very small file cabinets to begin with.