I know we saw a ton of them after 2007/08 crash but how are these sellers ending up underwater in our current market? 

It took forever for the market to make it not underwater from the crash. 

Those indicators that showed the crash last time are watched today.

So the issue is that prior to knowing about this I got under contract with the mindset that I will be working with the company that I am with now until I pay off the 401k loan off and now that I know about this other job opportunity which is a probability that I might get my questions is would it be better just to withdraw the money from my 401k and get penalize and taxed or just go ahead with the loan and if I get offered the new job just take the hit later when tax season arrives.

For example, how would an apartment building lose its value in a market crash, and be tough sell?

While looking over the letter, my GC arrived and started talking to the inspector.

So make sure your numbers are good in that area.It could still become a profitable rental by not taking out ALL the equity. ie, buy some cash flow.The BRRRR method is likely less doable now since property prices have recovered from the crash discount bargain basement prices they were at for several years.

I recently arrived in the US, on a visa.

I have been very fortunate since the crash to learn from a number of high quality investors such as @Chad Carson.

Something is going to crash and burn on you.